MEDICAL    tSCIHI©®L 


College  ofPfia^vMdo 


ESSENTIALS 

OF 


VOLUMETRIC  ANALYSIS 

AN  INTRODUCTION   TO    THE  SUBJECT,  ADAPTED 
TO    THE  NEEDS   OF  STUDENTS   OF  PHAR- 
MACEUTICAL  CHEMISTRY 


EMBRACING  THE    SUBJECTS    OF   ALKALIMETRY,   ACIDIMETRY,  PRECIPI- 
TATION   ANALYSIS,  OXIDIMETRY,    INDIRECT    OXIDATION,  lODOM- 
ETRY,  ASSAY   PROCESSES   FOR   DRUGS,  ESTIMATION  OF  AL- 
KALOIDS,   PHENOL,    SUGARS,    THEORY,    APPLICA-       ' 
TION   AND    DESCRIPTION    OF    INDICATORS 


BY 

HENRY  W.  SCHIMPF,  Ph.G.,  M.D. 

Professor  of  Analytical  Chemistry  in  the  Brooklyn  College  of  Pharmacy 

California  CoJIoge  of  Pharmaes 


THIRD  EDITION— REWRITTEN  AND  ENLARGED 
.  .TOTxAtL  ISSUJJ,  ^IX  THOUSAND  ;  ^  ^ 


JOHN  WILEY  &  SONS,   Inc. 
London:    CHAPMAN  &  HALL,  Limited 


Copyright,  1903,  1911,  1917 

BY 

HENRY  W.  SCHIMPF 


1 1 .»:  /•\*, '    ':  '...*     v'  : 

•  •    •        ^ 

•  t  •    •       • 

•  •  •    •       • 

>•    •    •        • 

;*•,•*',  «*•'•.  ,•.'*•      •  •*♦*• .   •  I 

«       •       ••    •   4    • 

PRESS  OF 

BRAUNWORTH   &   CO. 

BOOK   MANUFACTURERS 

BROOKLYN,    N.   Y. 

PREFACE  TO  THE  THIRD  EDITION 


The  exhaustion  of  the  second  edition  and  the  appearance 
of  the  new  United  States  Pharmacopoeia  as  well  as  the  increas- 
ing demand  for  the  book  have  necessitated  a  complete  revision 
of  ''  Essentials  of  Volumetric  Analysis,"  and  its  issuance  as  a 
third  edition. 

The  book  has  been  considerably  improved  by  the  introduc- 
tion of  many  new  assay  processes,  among  which  may  be  men- 
tioned the  assays  of  mercuric  salts,  phosphates  and  hypophos- 
phites  by  means  of  standard  sulphocyanate  solution;  assays  of 
chlorates,  perborates,  chloral,  acetone,  resorcinol,  phenyl- 
sulphonates,  arsenates,  and  alkali  cacodylate. 

The  "ex."  has  been  replaced  by  "  mil "  and  in  other  re- 
spects changes  are  made  to  accord  with  the  new  Pharmacopoeia. 

Henry  W.  Schimpf. 

New  York  City, 
October,  191 7. 

iii 


42118 


Digitized  by  the  Internet  Archive 

in  2007  with  funding  from 

IVIicrosoft  Corporation 


http://www.archive.org/details/essentialsofvoluOOschirich 


PREFACE  TO  THE  FIRST  EDITION 


Tke  growing  need  for  a  short  text-book  which  will  make 
the  principles  of  volumetric  analysis  readily  available  without 
going  too  deeply  into  detailed  and  discursive  description  has 
led  to  the  preparation  of  this  elementary  treatise. 

In  the  following  pages  the  aim  is  to  present  the  principles 
of  this  interesting  and  important  subject  in  a  form  readily 
intelligible  to  students  and  available  for  lecture-room  and 
laboratory  work.  The  essential  points  are  condensed  within 
the  limits  of  a  small  book  with  the  intention  of  furnishing 
an  outline  which  may  serve  as  a  practical  guide  as  well  as 
an  introduction  to  the  more  advanced  and  voluminous  works 
on  the  subject.  , 

If  presented  in  a  suitable  manner  volumetric  analysis 
rarely  fails  to  prove  interesting  to  the  student,  because  it  gives 
him  a  clear  conception  of  the  quantitative  significance  of 
chemical  equations  and  thus  affords  practical  proofs  of  chemical 
laws;  it  furthermore  trains  the  student  to  make  careful  obser- 
vations, to  form  habits  of  accuracy  in  manipulation,  and  since 
the  processes  are  easily  carried  out,  enables  him  to  arrive 
readily  at  a  definite  numerical  conclusion. 

The  subject-matter  in  this  book  is  systematically  arranged 
as  far  as  can  be,  and  treated  as  concisely  as  is  consistent  with 
clearness  of  expression.  The  processes  are  grouped  under 
five  headings:  Neutralization,  Precipitation,  Oxidation,  Indirect 
Oxidation,  and  lodometry.     The  principles  underlying  each 


vi  PREFACE 

group  are  definitely  indicated,  and  their  application  illustrated 
by  numerous  practical  examples.  Other  subjects  treated 
include  methods  of  calibration  and  of  the  accurate  reading 
of  graduated  instruments,  the  calculation  of  the  results  of 
analyses,  the  preparation  and  standardization  of  volumetric 
solutions.  The  indicators,  their  selection  for  special  cases 
and  the  ionic  theory  regarding  their  action,  as  well  as  assay 
process  for  phenol,  sugars  and  vegetable  drugs  also  receive 
special  treatment.  The  author  hopes  that  he  has  prepared 
a  book  which  will  prove  serviceable  to  those  for  whom  it  was 
written  and  that  it  will  be  as  generously  received  as  were  the 
four  editions  of  his  Text-book  of  Volumetric  Analysis. 

Henry  W.  Schimpf. 


CONTENTS 


PAGE 

List  of  Elements  with  their  Atomic  Weights xii 

Table  of  Multiples  of  Atomic  Weights  and  Combinations.  . .   xiii 

CHAPTER  I 
Introduction i 

CHAPTER  II 

General   Principals   of   Chemical   Combination   upon   which 
Volumetric  Analysis  is  Based 4 

CHAPTER  III 

Volumetric  or  Standard  Solutions 7 

To  Titrate.     Residual  Titration. 

CHAPTER  IV 

Indicators 17 

The  Ionization  Theory.     The  Ionization  Theory  of  Indicators. 
A  Guide  for  the  Selection  of  Indicators. 

CHAPTER  V 
Apparatus  Used  in  Volumetric  Analysis 29 

CHAPTER  VI 

On  the  Use  of  Apparatus 40 

On  the  Reading  of  Instruments.     Calibration  of  Instruments. 

vii 


viii  CONTENTS 

CHAPTER  VII 

PAGE 

Methods  of  Calculating  Results 50 

Table  of  Normal  Factors,  etc.,  of  Alkalies,  Acids  and  Alkali 
Earths.  On  Stating  Results.  Table  of  Molecular  Weights  and 
Normal  Factors  for  the  most  Common  Oxids. 

CHAPTER  VHI 

Analysis  by  Neutralization 58 

Alkalimetry.  Preparation  of  Standard  Acid  Solutions.  Esti- 
mation of  AlkaH  Hydroxids.  Estimation  of  Alkali  Carbonates. 
Mixed  Alkali  Hydroxid  and  Carbonate.  Estimation  of  Alkali 
Bicarbonates  when  Mixed  with  Carbonates.  Sodium  Borate. 
Sodium  Cacodylate.  Sodium  Glycerophosphate.  Estimation 
of  Organic  Salts  of  the  Alkahes.  Table  of  Normal  Factors, 
etc.,  of  the  Organic  Salts  of  the  Alkalies.  Estimation  of  Salts 
of  the  AlkaH  Earth  Metals.  Estimation  of  Mixed  Hydroxids 
and  Carbonates  of  Alkali  Earths.  Acidimetry.  Estimation 
of  Acids.  Preparation  of  Standard  Alkali  Solutions.  Table 
Showing  Quantity  to  be  taken  for  Analysis  in  Direct  Percentage 
Estimations. 

CHAPTER  IX 

Analysis  by  Precipitation 113 

Preparation  of  Decinormal  Silver  Nitrate,  Decinormal  Sodium 
Chlorid,  and  Decinormal  Sulphocyanate.  Estimation  of  Soluble 
Haloid  Salts.  Mohr's  Method  with  Chromate  Indicator.  Ti- 
tration without  an  Indicator.  Estimation  of  Haloid  Acids. 
Assay  of  Phosphoric  Acid.  Estimation  of  Cyanogen.  Esti- 
mation of  Silver  Salts.  Estimation  of  MetalHc  Silver  and 
Silver  Alloys.  Assay  of  Mercuric  Salts.  Table  of  Substances 
Estimated  by  Precipitation. 

CHAPTER  X 

Analysis  by  Oxidation  and  Reduction 139 

Preparation  of  Decinormal  Potassium  Permanganate. 

Volumetric  Analyses  by  Means  of  Potassium  Permanganate. 
On  the  Use  of  Empirical  Permanganate  Solution.  Typical 
Analyses  by  Permanganate.     Direct  Titrations.     Estimation  of 


CONTENTS  ix 

PAGE 

Ferrous  Salts.  Estimation  of  Metallic  Iron  in  Ferrum  Reduc- 
tum.  Estimation  of  Oxalic  Acid  and  Oxalates,  Estimation 
of  Calcium.  Estimation  of  Hydrogen  Dioxid,  Barium  Dioxid, 
and  Sodium  Perborate.  Estimation  of  Ferric  Salts.  Estima- 
tion of  Nitrous  Acid  and  Nitrites.  Residual  Titrations.  Esti- 
mation of  Hypophosphorous  Acid  and  Hypophosphites.  Esti- 
mation of  Calcium  Salts.  Estimation  of  Lead  Acetate  and 
Subacetate.  Estimation  of  Manganese  Dioxid.  Estimation 
of  Nitrates,  Chlorates,  Chromates,  and  Chromic  Acid.  Estima- 
tion of  Tin.    Estimation  of  Copper. 

Volumetric  Analysis  by  Means  of  Potassium  Bichromate. 
Preparation  of  Decinormal  Potassium  Bichromate.  Estima- 
tion of  Ferrous  Salts.  Table  of  Substances  Estimated  by 
Permanganate  or  Bichromate. 

Analysis  by  Indirect  Oxidation.  Preparation  of  Becinormal 
lodin.  Starch  Solution.  On  the  Use  of  Sodium  Bicarbonate 
in  Titrations  with  lodin.  Estimation  of  Arsenous  Compounds. 
Estimation  of  Antimony  Compounds.  Estimation  of  Sul- 
phurous Acid  and  Sulphites.  Estimation  of  Sodium  Thiosul- 
phate.  Hydrogen  Sulphid  and  Sulphids.  Table  of  Substances 
Estimated  by  Means  of  lodin  Solution. 

Estimation  of  Substances  Readily  Reduced.  lodometry.  Esti- 
mations Involving  the  Use  of  Sodium  Thiosulphate  V.S.  Prep- 
aration of  Becinormal  Thiosulphate  V.S.  Estimation  of  Free 
lodin.  Indirect  lodometric  Estimations.  Estimation  of  Free 
Chlorin  or  Bromin.  Estimation  of  Available  Chlorin.  Esti- 
mation of  Hydrogen  Bioxid.  Bistillation  Methods.  Estima- 
tion of  Manganese  Bioxid.  Chromic  Acid  and  Chromates. 
Estimation  of  Alkali  lodids.  Bigestion  Methods.  Estimation 
of  Chlorates,  Bromates  and  lodates.  Estimation  of  Ferric 
Salts.  Estimation  of  Chromates,  Arsenates,  Antimonic  Salts, 
Copper,  and  Mercury. 

Chlorometry,  Reduction  Methods,  Involving  the  Use  of  Arsenvus 
Acid  Solutions.  Preparation  of  Standard  Arsenous  Acid  V.S. 
Iodized  Starch  Test  Paper.  Estimation  of  Free  Halogens. 
Estimation  of  Available  Chlorin.  Chlorometric  Assay  of 
Manganese  Bioxid. 


X  CONTENTS 

PAGE 

Reduction  Methods  Involving  the  Use  of  Stannous  Chlorid  V.S. 
Estimation  of  Iron  by  Means  of  Stannous  Chlorid.  Estima- 
tion of  Mercuric  Salts. 

PART   II 
CHAPTER  XI 

Estimation  of  Alkaloids  .  . 251 

Table  of  Factors  for  Alkaloids.  Gordin's  Modified  Alkali- 
metric  Method  for  Titrating  Alkaloids. 

CHAPTER  XII 

Assaying  of  Vegetable  Drugs  and  their  Preparation 259 

Separation  of  Alkaloids  and  Use  of  Immiscible  Solvents. 
Kebler's  Modification  of  the  Keller  Method.  Assay  of  Galenical 
Preparations.    Lloyd's  Method.     Katz's  Method. 

CHAPTER  XIII 

Estimations  Involving  Use  of  Decinormal  Bromin  V.S 271 

Preparation  of  Decinormal  Bromin  Solution.  Assay  of 
Phenol.     Resorcinol.     Phenylsulphonates. 

CHAPTER  XIV 

Some  Technical  Methods  for  Fats,  Oils  and  Waxes 277 

The  Acid  Value.  The  Saponification  Number.  Volatile 
Fatty  Acid  Number.  Reichert's  Number.  The  Reichert- 
Meissl  Number.  Hiibl's  Number.  Hanus'  Number.  Acid 
Number  for  Resins. 

CHAPTER  XV 

Estimation  of  Sugars 291 

Preparation  of  FehHng's  Solution.  Determination  of  the 
End-point.  Estimation  Starch  after  Inversion.  Estimation 
of  Maltose  in  Malt  Extracts.  Estimation  of  Diastasic  Value 
of  Malt  Extract. 


CONTENTS  xi 

CHAPTER  XVI 

PACK 

Estimation  of  Formaldehyde 299 

The  Ammonia  Method.  The  Ammonium  Chlorid  Method. 
Oxidation  Method  by  Means  of  Hydrogen  Dioxid.  The  lodo- 
metric  Method.  The  Cyanid  Method.  Assay  of  Paraformal- 
dehyde and  Acetone. 

CHAPTER  XVH 

Estimation  of  Alcohol  in  Tinctures  and  Beverages 308 

Alcoholometric  Table. 

PART  III 

CHAPTER  XVIII 

The  Nitrometer 313 

The  Law  of  Charles.     The  Law  of  Boyle. 

CHAPTER  XIX 

Assay  of  Nitrites , 318 

Spirit  of  Nitrous  Ether.  Amyl  Nitrite.  Sodium  Nitrite. 
Nitric  Acid  in  Nitrates. 

CHAPTER  XX 

Hydrogen  Dioxid 323 

Use  of  Nitrometer.  Use  of  Urea  Apparatus.  The.  Hypo- 
chlorite Method.  The  Hypobromite  Method.  Table  Showing 
Weight  in  Milhgrams  of  H2O2  corresponding  to  one  cc.  of 
Moist  Oxygen. 

CHAPTER  XXI 
Estimation  of  Soluble  Carbonates  by  the  Use  of  the  Nitrom- 
eter    328 

CHAPTER  XXII 

Estimation  of  Urea  in  Urine 329 

The  Doremus'  Ureometer.  The  Hinds-Doremus  Ureometer. 
Squibb 's  Urea  Apparatus. 

APPENDIX 

Description  of  Indicators  Alphabetically  Arranged 334 


A   LIST   OF  THE  MORE  COMMON  ELEMENTS  WITH  THEIR 
SYMBOLS  AND  ATOMIC  WEIGHTS 


Atomic  Weight* 
based  on  0  =  i6. 


Atomic  Weight 
based  on  H  =  i . 


Approximate 
Atomic  Weight. 


Aluminium Al. 

Antimony Sb. 

Arsenic As. 

Barium Ba. 

Bismuth Bi. 

Boron B, 

Bromin Br. 

Cadmium Cd. 

Calcium g. ..... .  Ca. 

Carbon .W...Y...    C. 

Chlorin.   ......i. CI. 

Chromium /. Cr. 

Cobalt /. Co. 

Copper ' Cu. 

Fluorin - F. 

Gold Au. 

Hydrogen 11. 

lodin I. 

Iron Fc. 

Lead Tb. 

Lithium Li. 

Magnesium Mg. 

Manganese Mn. 

Mercury Hg 

Molybdenum Mo 

Nickel Ni 

Nitrogen N 

Oxygen O 

Phosphorus P. 

Platinum Pt. 

Potassium K. 

Silver Ag. 

Sodium Na. 

Strontium Sr. 

Sulphur S. 

Tin Sn. 

Zinc Zn. 


27.1 
120.2 
74.96 

137-37 

208.0 
II  .0 
79.92 

1 1 2 . 40 
40.07 
12.005 
35  46 
52.0 
58 -97 
63  57 
19.0 

197.2 
1.008 

126.92 
55-84 

207 . 20 

6.94 

24-32 

54-93 

200.6 
96.0 
58.68 
14.01 
16.00 
31.04 

195-2 

39  I 

107.88 
23.00 

87-63 
32.06 
118. 7 
65 -37 


26.9 

119-3 
74-3 

136.4 

206.4 
10.9 
79-36 

III  .6 
39-8 
II  .91 
35-18 
51-7 
58.56 
63.1 
18.9 

195-7 
1 .000 

125.9 

55-5 
205.35 
6.98 

24.18 

54.6 
198.5 

95-3 

58.3 

13-93 

15.88 

30-77 
193.3 

38.86 
107.12 

22.88 

86.94 

31-83 
118. 1 
64.9 


27.0 

120.0 

75.0 

136.0 

206.0 

II  .0 

80.0 

III  .0 

40.0 

12.0 

35-5 
52.0 
58.0 
63  o 
19.0 

196.0 
1 .0 

126.0 
56.0 

206 
7.0 
24.0 

55.0 
200.0 

95 -o 
58.0 
14.0 
16.0 
31.0 

194.0 
39-0 

107.0 
23.0 
87.0 
32.0 

118. o 
65  -O 


International  Atomic  Weights,  1916. 


XU 


TABLE    OF    MULTIPLES    OF    SOME    ATOMIC    WEIGHTS 
COMBINATIONS  IN  FREQUENT  USE. 


AND 


I 

2 

3 

4 

5 

6 

7 

8 

9 

H 

1.008 

2.016 

3.024 

4.032 

5.040 

6.048 

7-056 

8.064 

9.072 

O 

16.000 

32. 

48. 

64. 

80. 

96. 

1x2. 

128. 

144. 

OH 

17.008 

34.016 

51.024 

68.032 

85.04 

102.048 

119.056 

136.064 

153.072 

H2O 

18.016 

36.032 

54.048 

72.064 

90.08 

108.096 

126. 112 

144.128 

162.144 

N 

14.01 

28.02 

42.03 

56.04 

70.05 

84.06 

98.07 

112.08 

126.09 

NH3 

17.034 

34  064 

51.102 

68.128 

85.170 

102.204 

119.238 

136.272 

153.306 

NH4 

18.042 

36.084 

54.126 

72.168 

90.210 

108. 2S2 

126.294 

144-336 

162.378 

NO2 

46.01 

92.02 

138.03 

184.04 

230.0s 

276.06 

312.07 

388.08 

414.09 

NO3 

62.01 

124.02 

186.03 

248.04 

310.05 

372.06 

434-07 

496.08 

558.09 

C 

12.00 

24. 

36. 

48. 

60. 

72. 

84. 

96. 

108. 

CO2 

44.00 

88. 

132. 

176. 

220.' 

264. 

308. 

352. 

396. 

CO3 

60.00 

120. 

180. 

240. 

300. 

360. 

420. 

480. 

540. 

CN 

26.01 

52.02 

78.03 

104.04 

130.05 

156.06 

182.07 

208.08 

234-09 

CI 

35.46 

70.92 

106.38 

141.84 

177.3 

212.76 

248.22 

283.68 

319.14 

Br 

79.92 

159.84 

239.76 

319.68 

399.6 

479.52 

559.44 

639.36 

719.28 

I 

126.92 

253.84 

380.76 

507.68 

634.6 

761.52 

888.44 

1015.36 

1142.28 

S 

32.07 

64.14 

96.21 

128.28 

160.35 

192.42 

224.49 

256.56 

288.63 

SO3 

80.07 

160.14 

240.21 

320.28 

400.35 

480.42 

560.49 

640 . 56 

720.63 

SO4 

96.07 

192.14 

288.21 

384.28 

480.3s 

586.42 

672.49 

768.56 

864-63 

PO4 

95.04 

190.08 

285.12 

380.16 

475.20 

580.24 

665.28 

760.32 

855.36 

P2O6 

142.08 

•284.16 

426.24 

568.32 

710.40 

852.48 

994-56 

1136.64 

1278.72 

Na 

23.00 

46. 

69. 

92. 

115. 

138. 

161. 

184. 

207. 

K 

39.10 

78.20 

117.30 

156.40 

195.50 

234.60 

273.70 

312.80 

351-90 

Li 

6.94 

13.88 

20.82 

27.76 

34-70 

41.64 

48.  S8 

55-52 

62.46 

Ca 

40.07 

80.14 

120.21 

160.28 

200.35 

240.42 

280.49 

320.56 

360.63 

Ba 

137.37 

274-74 

412. II 

549.48 

686.85 

824.22 

761.59 

898.96 

1036.33 

Ag 

107.88 

215. 76 

323.64 

431.52 

539.40 

647.28 

755.16 

863 . 04 

.970.92 

Fe 

55.84 

III. 68 

167.52 

223.36 

279-20 

335-04 

390.88 

446.72 

502.56 

Hg 

200.6 

401.2 

601.8 

802.4 

1003.0 

1203.6 

1404.2 

1604.8 

1805. 4 

ABBREVIATIONS  AND  SIGNS 

cc cubic  centimeter 

gm gramme,  15.43235  grains 

gr grain 

at.  wt atomic  weight 

V.S volumetric  solution 

T.S test  solution. 

U.  S.  P United  States  Pharmacopoeia. 

— normal. 

I 

— decinormal. 

10 

— centinormal. 

100 

— semi-normal. 

2 

2 
;rT  or  2N double-normal. 

N 

mil milliliter.  xiv 


THE  ESSENTIALS  OF  VOLUMETRIC 
ANALYSIS 


CHAPTER  I 
INTRODUCTION 

In  a  chemical  analysis  the  aim  is  to  determine  the  nature 
af  the  chemical  substances  contained  in  a  given  compound 
or  to  ascertain  their  quantities.  In  the  former  case  the 
analysis  is  a  qualitative,  in  the  latter  a  quantitive,  one. 

The  quantitive  analysis  of  a  substance  may  be  made 
either  by  the  gravimetric  or  the  volumeiric  method. 

The  Gravimetric  Method  consists  in  separating  and 
weighing  the  constituents  either  in  their  natural  states  or  in 
the  form  of  new  and  definite  compounds,  the  composition 
of  which  is  known  to  the  analyst.  From  the  weights  of 
these  new  compounds  the  analyst  can  calculate  the  quantities 
of  the  original  constituents. 

Example.  To  determine  the  quantity  of  silver  in  a  solu- 
tion by  the  gravimetric  method  we  proceed  as  follows : 

Ten  grams  of  a  solution  containing  silver  in  the  form  of 
silver  nitrate  (AgNOs)  is  placed  into  a  beaker,  and,  after 
slightly  acidulating  with  nitric  acid,  is  treated  with  hydro- 
chloric acid,  drop  by  drop,  until  no  further  precipitation 
occurs.  The  precipitate  which  consists  of  silver  chlorid  (AgCl) 
is  then  separated  by  filtration,  thoroughly  washed,  dried  and 
weighed.  Its  weight  is  found  to  be  0.69  gm.  The  calcu- 
lation is  then  made  as  follows:    143.34  gms.  of  silver  chlorid 


2  THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

represents  107.88  gms.  of  silver  or  169.89  gms.  of  silver  nitrate, 
as  the  equation  shows: 

AgNOs  +  HCl  =  AgCl  +  HNO3. 

169.89  143.34 

Therefore,  0.69  gm.  of  silver  chlorid  will  represent 
107.88, 


143-34 


Xo.69  =  o.5i9  gm.  of  silver, 


t6q.8q        .  r.  p    .,  . 

or  X 0.60  =  0.8 1 7  2jm.  of  silver  nitrate. 

143-34 

The  Volumetric  Method.  This  method  depends  upon  the 
use  of  solutions  {standard  solutions)  which  are  of  known  strength 
and  paying  attention  to  the  valume  of  such  a  solution  which 
must  be  added  to  the  substance  under  analysis  to  perform 
with  it  and  complete  a  certain  reaction.  Thus,  if  we  conduct 
an  analysis  by  means  of  such  a  solution,  and  can  express  by 
a  chemical  equation  the  reaction  which  takes  place,  we  can 
readily  and  accurately  calculate  the  quantity  present  of  the 
substance  under  analysis. 

Example.  If  a  silver  solution  is  to  be  analyzed  by  this 
method  it  is  treated  with  a  standard  solution  of  sodium  chlorid, 
added  slowly  from  a  burrette  until  no  more  silver  chlorid  is 
precipitated.  Each  mil  of  this  standard  solution  will  precip- 
itate a  certain  weight  of  silver  as  silver  chlorid,  and  hence 
by  noting  the  number  of  mils  used  to  complete  the  precipitation, 
the  weight  of  the  silver  in  the  solution  analyzed  is  easily 
ascertained. 

N 
The  —  sodium  chlorid  solution  is  generally  used  for  this 

purpose.  It  is  made  by  dissolving  iV  of  the  molecular  weight 
of  the  salt  (in  grams)  (5.846  gms.)  in  sufficient  water  to 
make  1000  mils.  1000  mils  of  this  solution  will  precipitate  -^ 
of  the  atomic  weight  of  silver  (in  grams)  (10.788  gms.),  and 


INTRODUCTION  3 

hence    each   mil   of   the   sodium   chlorid   solution   represents 

0.010788  gm.  of  metallic  silver,  and  by  multiplying  this  figure 

by  the  number  of  mils  of    the  sodium  chlorid  solution  used, 

the  quantity  of  silver  in  the  solution  under  analysis  is  ascer- 

N 
tained.     If  in  the  above  analysis  100  mils  of    the  ■ —  sodium 

•'  10 

chlorid  solution  were  used,  then  0.010788X100=1.0788  gms. 
of  metallic  silver. 

The  reaction  is  illustrated  by  this  equation : 

AgNOa   +   NaCl   =   AgCl   +   NaNOs. 

10)107.88      10)58.46 

1000)  10.788  gms.  1000)  5.846  gms.  =  1000  mil  —  V.S. 

•  10 

0.010788  gm.  0.005846  gra.  =  I  mil  "     " 

From  the  examples  given  it  will  be  seen  that  the  gravi- 
metric operations  consume  no  little  time,  and  require  the 
exercise  of  considerable  skill.  The  washing  of  the  precipitate 
must  be  thoroughly  performed  in  order  that  it  be  freed  from 
all  adhering  matter.  The  drying  also  is  a  matter  of  some 
consequence  and  must  be  performed  in  such  a  manner  as 
to  prevent  the  admixture  of  dust  or  the  decomposition  of 
the  precipitate  by  excessive  heat.  A  very  accurate  balance 
is  also  required. 

The  volumetric  operations,  on  the  other  hand,  do  not 
require  that  the  substance  to  be  determined  be  separated  in 
the  form  of  a  compound  of  known  composition  and  weighed 
in  the  dry  state;  in  fact,  the  substance  may  be  accurately 
estimated  when  mixed  with  many  others.  It  therefore  obviates 
the  necessity  for  the  frequent  separations  and  weighings  which 
the  gravimetric  method  demands,  and  enables  the  analyst  to 
do  the  work  in  a  very  short  time. 

The  instruments  needed  for  volumetric  work  are  few  and 
simple,  and  comparatively  little  skill  is  required.  Further- 
more, the  results  obtained  are  in  most  instances  more  accurate. 


CHAPTER  II 

GENERAL   PRINCIPLES   OF   CHEMICAL   COMBINATION 
UPON  WHICH  VOLUMETRIC  ANALYSIS  IS  BASED 

I.  When  substances  unite  chemically  the  union  always 
takes  place  in  definite  and  invariable  proportions.  Thus  when 
silver  nitrate  and  sodium  chlorid  are  brought  together,  169.89 
parts  (by  weight)  of  silver  nitrate  and  58.46  parts  (by  weight) 
of  sodium  chlorid  will  react  with  each  other,  producing  143.34 
parts  of  a  curdy  white  precipitate  (silver  chlorid). 

These  substances  will  react  with  each  other  in  these  pro- 
portions only. 

If  a  greater  proportion  of  silver  nitrate  than  that  above 
stated  be  added  to  the  sodium  chlorid,  only  the  above  pro- 
portion will  react,  the  excess  remaining  unchanged. 

The  same  is  true  if  sodium  chlorid  be  added  in  excess 
of  the  above  proportions.  For  instance,  if  200  parts  of  silver 
nitrate  be  mixed  with  5S.46  parts  of  sodium  chlorid,  169.89 
parts  only  will  react  with  the  sodium  chlorid,  while  30.11 
parts  of  silver  nitrate  will  remain  unchanged.  Again,  when 
potassium  hydroxid  and  sulphuric  acid  are  mixed  potassium 
sulphate  is  formed,  112.2  parts  of  potassium  hydroxid  and 
98.1  parts  of  sulphuric  acid  being  required  for  complete 
neutralization.  These  two  substances  unite  chemically  in  these 
proportions  only. 

The  equation  is 

2KOH-hH2S04=K2S04  +  2H20. 


GENERAL  PRINCIPLES  OF  CHEMICAL  COMBINATION     5 

In  other  words,  112.2  parts  of  KOH  will  neutralize  98.1 
parts  of  H2SO4,  and  consequently  98.1  parts  of  H2SO4  will 
neutralize  112.2  parts  of  KOH. 

Oxalic  acid  and  sodium  carbonate  react  upon  each  other 
in  the  proportions  shown  in  the  equation 


H2C2O4  •  2H2O  +  Na2C03  =  Na2C204  +  CO2 + 3H2O. 

126.05  106 


126.05  parts  of  crystallized  oxalic  acid  are  neutralized  by 
106  parts  of  anhydrous  sodium  carbonate. 

2.  Definite  chemical  compounds  always  contain  the  same 
elements  in  exactly  the  same  proportions,  the  proportions 
Being  those  of  their  atomic  weights,  or  some  multiple  of  these 
weights. 

Thus  sodium  chlorid  (NaCl)  contains  23  parts  of  metallic 
sodium  and  35.46  parts  of  chlorin,  these  being  the  atomic 
weights  of  sodium  and  chlorin  respectively. 

Potassium  sulphate  (K2SO4)  contains  twice  39.1  =  78.2 
parts  of  potassium,  32.07  parts  of  sulphur,  and  four  times 
16  =  64  parts  of  oxygen. 

Potassium  hydroxid  (KOH)  contains  39.1  parts  of  potas- 
sium, 16  parts  of  oxygen,  and  one  part  of  hydrogen.  Hydro- 
chloric acid  (HCl)  contains  one  part  of  hydrogen  and  35.46 
parts  of  chlorin. 

Upon  these  facts  the  volumetric  methods  of  analysis  are 
based. 

It  has  been  shown  that  98.1  gms.  of  sulphuric  acid  will 
neutralize  112.2  gms.  of  potassium  hydroxid.  It  is  therefore 
evident  if  a  solution  of  sulphuric  acid  be  made  containing 
49.05  gms.  of  the  pure  acid  in  icoo  mils,  that  one  mil  of  this 
solution  will  neutralize  0.0561  gm.  of  potassium  hydroxid. 
In  estimating  alkalies  with  this  acid  solution,   the  latter  is 


6  THE  ESSENTIALS  OF  VOLUMETRIC   ANALYSIS 

added  from  a  burette,  in  small  portions,  until  the  alkali  is 
neutralized,  as  shown  by  its  reaction  with  some  indicator. 

Each  mil  of  the  acid  solution  required  before  neutralization 
is  complete  indicates  0.0561  gm.  of  KOH,  and  the  number 
of  mils  used  multiplied  by  0.0561  gm.  gives  the  quantity  of 
pure  KOH  in  the  sample  analyzed. 

One  mil  of  the  same  solution  will  neutralize  0.04  gm.  of 
sodium  hydroxid  (NaOH),  0.053  gm.  of  anhydrous  sodium 
carbonate  (Na2C03),  etc. 

If  a  solution  of  crystallized  oxalic  acid  be  made  by  dis- 
solving 63.02  gms.  in  sufficient  water  to  make  icoo  mils,  we 
will  have  a  normal  solution,  the  neutralizing  power  of  which 
is  exactly  equivalent  to  the  above  mentioned  normal  sulphuric 
acid  solution. 

The  strength  of  acids  is  estimated  by  alkali  volumetric 
solutions.  A  normal  solution  of  potassium  hydroxid  containing 
56.1  gms.  in  the  liter  will  neutralize  exactly  i  liter  of  the 
normal  acid  solution;  imil  of  this  normal  alkaliiwill  neutralize 
0.03646  gm.  of  HCl,  0.06362  gm.  of  H2C204,or  0.04905  gm. 
of  H2SO4,  etc. 


CHAPTER  III 
VOLUMETRIC  OR  STANDARD  SOLUTIONS 

Any  solution  employed  in  volumetric  analysis  for  the 
purpose  of  estimating  the  strength  of  substances,  that  is,  any 
solution  the  chemical  power  or  titer  of  which  has  been  deter- 
mined, is  designated  a  standard  or  volumetric  solution.  Such 
a  solution  is  said  to  be  "titrated"  (French  titre  =  title  or 
power),  and  is  sometimes  also  called  a  set  solution  or  a  stand- 
ardized solution.  It  may  be  normal,  decinormal,  empirical, 
or  of  any  strength,  so  long  as  its  .strength  is  known. 

When  volumetric  analysis  first  came  into  use  the  solutions 
were  so  made  that  each  substance  to  be  estimated  had  its 
own  special  volumetric  solution,  and  this  was  usually  of  such 
strength  as  to  give  the  result  in  percentages.  Thus  a  certain 
strength  of  standard  acid  was  employed  for  potash,  another 
for  soda,  and  a  third  for  ammonia,  and  in  testing  the  acids, 
each  had  its  own  special  standard  alkali.  These  solutions 
were  known  as  normal  solutions;  they  are  still  to  some  extent 
in  use,  and  since  solutions  now  designated  as  normal  are  of 
an  entirely  different  character,  it  is  important  that  no  miscon- 
conception  should  exist  when  a  normal  solution  is  spoken  of. 

Normal  Solutions  are  those  which  contain  one  liter  (looo 
mils),  the  molecular  weight  of  the  active  reagent  expressed  in 
grams,  and  reduced  to  the  valency  corresponding  to  one  atom 
of  replaceable  hydrogen  or  its  equivalent.  In  other  words, 
those  which  contain  in  looo  mils  in  any  given  reaction  the 
chemical  equivalent  of  one  gram  of  hydrogen.  The  now  ac- 
cepted   basis  for  the  atomic  weights  0  =  i6  makes  a  slight 

7 


8  THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

• 

change  in  the  quantity  of  reagent  in  a  normal  solution,  i.e., 
it  will  contain  in  looo  mils  the  exact  equivalent  of  8  gms.  of 
oxygen. 

Thus  in  univalent  or  monobasic  compounds  the  full  molec- 
ular weight  in  grams  is  contained  in  a  liter  of  the  normal  solu- 
tion. 

Example.  Hydrochloric  acid,  HCl,  having  one  replaceable 
hydrogen  atom,  its  normal  solution  would  contain  the  full 
molecular  weight,  36.46  gms.  in  a  liter.  A  normal  solution  of 
potassium  hydroxid  should  contain  56.1  gms.  of  KOH  in  a 
liter,  while  that  of  sodium  hydroxid  should  contain  40  gms.  of 
absolute  NaOH. 

Normal  solutions  of  bivalent  or  dibasic  compounds  contain 
in  1000  mils  one-half  the  molecular  weight  in  grams.  Thus, 
oxalic  acid,  H2C2O4  +  2H2O  =  126.05,  having  two  replaceable  H 
atoms,  one-half  of  its  molecular  weight' in  grams  =  63.02  is 
contained  in  a  liter  of  its  normal  solution.  For  the  same 
reason  a  liter  of  a  normal  solution  of  sulphuric  acid  contains 

o 

——  =  49.05  gms.,  and  a  liter  of  a  normal  solution  of  sodium 

106 
carbonate  Na2C03  contains  —  =  53  gms.,  while  in  the  case 

of  trivalent  or  tribasic  compounds  one-thu'd  of  the  molecular 
weight  in  grams  is  contained  in  a  liter  of  the  normal  solution. 
Thus  it  will  be  seen  that  one  mil  of  any  normal  acid  solution 
will  neutralize  one  mil  of  any  normal  alkali  solution,  because  • 
one  molecule  of  a  univalent  acid  will  neutralize  one  molecule 
of  a  univalent  alkali,  or  a  half  a  molecule  of  a  bivalent  alkali. 
This  is  shown  by  the  equations 

HCl  +  NaOH  =  NaCl-hH20, 

36.46         46 

2HCI  +  Na2C03  =  2NaCl  -h  H2O  -H  CO2. 

2)72.92    2)106 
36.46        53 


VOLUMETRIC   OR   STANDARD   SOLUTIONS  9 

The  value  of  a  reagent  as  expressed  by  its  hydrogen 
equivalent  is  readily  seen  in  the  case  of  acids  and  alkalies 
by  reference  to  the  chemical  formula,  but  in  such  standard 
solutions  as  potassium  dichromate,  potassium  permanganate, 
sodium  thiosulphate,  and  others,  the  particular  reaction  in 
any  given  analysis  must  be  taken  into  account  in  making  a 
normal  solution;  for  instance,  when  K2Cr207  is  to  be  used 
as  a  precipitating  agent,  its  reaction  is  as  follows : 

2Ba(C2H302)2  +  K2Cr207  +  H2O  =  2BaCr04  +  2KC2H3O2 

+  2HC2H3O2. 

It  is  thus  seen  that  one  molecule  of  K2Cr207  will  cause 

the   precipitation   of   two   atoms   of   barium   in   the   form   of 

chromate.     Each  atom  of  barium  is  chemically  equivalent  to 

two  atoms  of  hydrogen;    therefore  one-fourth  of  a  molecule 

of   K2Cr207  is   equivalent   to  one  atom  of  hydrogen.     And 

therefore   a   normal   solution   of   this   salt,   when   used   as   a 

precipitating  agent,  must  contain  in  one  liter  one-fourth  of 

294.2 
its  molecular  weight  in  grams; =  73-55  gms. 

If  K2Cr207  is  to  be  used  as  an  oxidizing  agent,  the  three 
atoms  of  oxygen  which  it  yields  for  oxidizing  purposes  must 
be  taken  into  account.  When  this  salt  oxidizes  it  splits  up 
into  K2O -f- Cr203  +  O3.  The  three  atoms  of  oxygen  combine 
with  and  oxidize  the  salt  acted  upon,  or  they  combine  with 
an  equivalent  quantity  of  the  hydrogen  of  an  acid  and  liberate 
the  acidulous  part,  which  then  combines  with  the  salt.  As 
the  equations  show, 

6FeO  -f-  K2Cr207  =  K2O  H-  Cr203 + 3Fe203    or     (FeeOg) ; 

6FeS04  +  K2Cr207  +  7H2SO4  - 

7H2O  +  K2SO4  +  Cr2(S04)3  +3Fe2(S04)3; 
7H2SO4  +  K2Cr207  -=  3SO4  +  7H2O  -f-  K2SO4  +  Cr2(S04)3. 


10        THE   ESSENTIALS   OF   VOLUMETRIC  ANALYSIS 

Each  of  these  atoms  of  oxygen  are  equivalent  to  two  atoms 
of  hydrogen.     Thus  O3  is  equivalent  to  He. 

Hence  a  liter  of  a  normal  solution  of  K2Cr207,  when  used 
as  an  oxidizing  agent,  contains  one-sixth  of  its  molecular 
weight  in  grams. 

The  same  may  be  said  of  potassium  permanganate  when 
used  as  an  oxidizing  agent. 

2KMn04  has  five  atoms  of  oxygen  which  are  available 
for  oxidizing  purposes,  and  each  of  these  is  capable  of  taking 
two  atoms  of  hydrogen  from  an  acid  and  liberating  the  acidulous 
part.  The  hydrogen  equivalent  of  this  salt  may  therefore  be 
said  to  be  one- tenth  of  the  weight  of  2KMn04,  and  a  normal 
solution  of  this  salt  contains  31.606  gms.  in  a  liter. 

Sodium  Thiosulphate  (Hyposulphite),  Na2S203,  is  another 
instance.  The  molecule  of  this  salt  has  two  atoms  of  sodium, 
which  have  replaced  two  atoms  of  hydrogen  of  thiosulphuric 
acid.  Thus  it  would  seem  that  a  normal  solution  should 
contain  one-half  of  the  molecular  weight  in  grams.  But  the 
particular  reaction  of  this  salt  with  iodin  is  taken  into  account. 

One  molecule  reacts  with  one  atom  of  iodin,  as  seen  in 
the  equation 

2Na2S203  + 12  =  2NaI  +  Na2S406. 

Since  iodin  is  univalent,  a  molecule  of  the  salt  is  equivalent 
to  one  atom  of  hydrogen. 

A   normal    solution    of    this    salt    therefore    contains    the 

molecular  weight  in  grams  in  a  liter. 

N 
Decinormal  Solutions,  — ,  are  one-tenth  the  strength   of 

normal  solutions. 

N 
Centinormal  Solutions,  — ,  are  one-hundredth  the  strength 

of  normal  solutions. 


VOLUMETRIC   OR   STANDARD    SOLUTIONS  11 

N 
Seminormal   Solutions,    — ,   are   one-half   the   strength   of 

normal  solutions. 

2 

Double-normal   Solutions,   ^,   are   twice   the   strength   of 

the  normal. 

Empirical  Solutions.  A  solution  which  does  not  contain 
an  exact  atomic  proportion  of  the  reagent  may  be  employed 
as  a  volumetric  solution  after  its  strength  or  titer  has  been 
determined.  Such  a  solution  is  said  to  be  empirical,  and 
solutions  of  this  sort  are  very  frequently  used.  To  prepare 
solutions  of  exactly  normal  strength  is  a  tedious  process  and 
often  inconvenient.  If  the  solution  is  approximately  normal 
and  its  strength  accurately  determined,  it  may  be  used  as  it 
is.  Again,  in  the  case  of  standard  solutions  of  the  caustic 
alkalies,  wJiich,  when  not  kept  with  all  precautions,  deteriorate 
readily  through  absorption  of  carbon  dioxid  from  the  air,  as  well 
as  through  their  action  upon  the  glass  containers.  To  restore 
the  titer  of  such  solutions  by  the  introduction  of  more  of  the 
alkali  is  an  unnecessary  waste  of  time,  inasmuch  as  it  is  only 
necessary  to  determine  its  exact  strength  and  then  use  it  as 
it  is»  For  instance,  if  an  approximately  normal  solution  of 
potassium  hydroxid  is  on  hand,  its  strength  is  determined  as 
follows ; 

Ten  mils  of  an  exactly  normal  oxalic  or  other" acid  solution 
are  put  into  a  beaker,  and  after  diluting  with  a  little  water, 
and  adding  three  or  four  drops  of  methyl  orange,  the  empirical 
potassium  hydroxid  solution  is  run  in  from  a  burette  until 
the  color  of  the  solution  changes  from  red  to  yellow;  the 
number  of  mils  required  is  then  noted. 

Assuming     that     10.4    mils    were    required    to    neutralize 

100 
the    10    mils   of   normal    acid,    hence    its    strength    is  —  or 

^  104 

0.9615,  that  of  a  strictly  normal  solution,  and  the  number  of 


12        THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

lOG 

mils    used    in   any    estimation    must   be    multiplied    by    — 

or  0.9615,  and  then  by  the  normal  factor  for  the  substance 
analyzed. 

It  is  a  good  plan  to  have  the  factor  marked  on  the  label 
of  the  bottle  containing  such  an  empirical  solution.  In  this 
case  it  would  be  X  0.961 5  =  normal. 

Standard  solutions  for  use  in  volumetric  analysis  are  usually 
solutions  of  acids,  bases,  or  salts,  and  in  two  cases  elements, 
namely,  iodin  and  bromin. 

A  standard  solution  of  a  base  is  usually  used  for  the  esti- 
mation of  free*  acids. 

A  standard  solution  of  an  acid  is  usually  used  for  the 
estimation  of  a  free  base,  or  the  basic  part  of  a  salt,  the  acid 
of  which  can  be  completely  expelled  by  the  acid  used  in  the 
standard  solution.     Example,  carbonates. 

A  standard  solution  of  a  salt  may  be  used  as  a  precipitant, 
or  it  may  be  used  as  an  oxidizing  or  reducing  agent. 

That  part  of  the  reagent  in  a  standard  solution  which 
reacts  with  the  substance  under  analysis  is  the  active  con- 
stituent of  the  solution.  As  Ag  in  AgNOs  is  the  active  con- 
stituent of  the  standard  solution  of  silver  nitrate, 

AgNOa  +  NaCl  ==  AgCl  +  NaNOg, 

or  CI  in  NaCl,  is.  the  active  constituent  of  the  standard  solution 
of  sodium  chlorid. 

If  the  reagent  is  a  base,  as  KOH,  the  basic  part  K  is  the 
active  constituent.  If  the  reagent  is  an  acid,  the  active  constit- 
uent is  the  acidulous  part,  as  SO4  in  H2SO4. 

If  the  action  of  the  reagent  is  oxidizing,  then  that  part 
of  the  reagent  which  produces  the  oxidation  is  the  active 
constituent. 

The -valence  of  an  acid  is  shown  by  the  number  of  replace- 


VOLUMETRIC  OR  STANDARD  SOLUTIONS  13 

able  hydrogen  atoms  it  contains.  Thus  HCl  is  univalent, 
H2SO4  is  bivalent,  which  means  that  a  molecule  of  HCl  is 
chemically  equivalent  to  one  atom  of  hydrogen,  and  a  molecule 
of  H2SO4  is  chemically  equivalent  to  two  atoms  of  hydrogen. 

The  valence  of  a  base  is  shown  by  the  number  of  hydroxyls 
it  is  combined  with.  As  KOIT  is  univalent,  Ca(OH)2  is 
bivalent. 

The  valence  of  a  salt  is  shown  by  the  equivalent  of  base 
which  has  replaced  the  hydrogen  of  the  corresponding  acid. 

Thus  NaCl,  in  which  Na  has  replaced  H  of  HCl,  is  uni- 
valent. 

R2SO4,  in  which  K2  has  replaced  H2  of  H2SO4,,  is  bivalent. 

Preparation  of  Volumetric  Solutions.  In  preparing  volu- 
metric solutions  it  must  be  borne  in  mind  that  most  salts  when 
dissolved  in  water  cause  a  condensation  in^  volume,  through 
reduction  of  temperature,  while  some  substances,  as  for  instance 
sulphuric  acid  and  alkali  hydroxids,  cause  a  rise  in  tempera- 
ture and  a  consequent  increase  in  volume.  It  is  therefore 
necessary,  aftel*  making  a  solution,  to  set  it  aside  for  a  short 
time,  in  order  to  allow  it  to  attain  the  proper  temperature 
before  measuring  it. 

It  is  always  the  best  plan  to  take  a  weighed  quantity  of 
the  salt,  slightly  greater  than  that  required  by  theory,  and 
to  dissolve  it  in  less  water  than  is  needed  for  the  finished 
solution.  This  solution  is  titrated,  its  strength  determined 
and  then  diluted  to  the  proper  measure. 

After  dilution  it  should  be  again  carefully  titrated  and 
its  titer  verified. 

All  volumetric  solutions  should  be  made  with  distilled 
water  and  with  reagents  which  are  of  a  high  degree  of  purity.  . 

Standard  Temperature.  A  cubic  centimeter  is  the  volume 
occupied  by  one  gram  of  distilled  water  at  its  maximum  density 
4°  C.   (39°  F.).     This,  however,  is  not  the  cubic  centimeter 


14        THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

used  in  volumetric  analysis.  It  is  convenient  to  use  in  analyses 
of  this  sort  a  cubic  centimeter  which  represents  the  volume  of 
one  gram  of  distilled  water  at  a  temperature  which  is  easily 
attained  and  maintained  at  any  season  of  the  year. 

The  temperature  at  which  measuring  instruments  are 
graduated  is  the  temperature  at  which  volumetric  solutions 
should  be  prepared,  and  at  which  all  volumetric  operations 
should  be  conducted.  4°  C.  is  a  temperature  at  which  it  is 
obviously  impossible  to  work  except  during  one  or  two  months 
of  the  year.  For  this  reason  the  temperature  of  16°  C.  (60.8°  F.) 
has  been  taken  as  the  standard.  The  cubic  centimeter  used 
in  volumetric  analysis,  under  this  standard,  is  the  volume 
occupied  by  one  gram  of  distilled  water  at  the  latter  tempera- 
ture. 

The  employment  of  this  standard  of  temperature,  though 
long  in  vogue,  is  justly  criticized  as  too  low.  In  order  to  obtain 
accurate  results  the  temperature  of  the  atmosphere  in  which 
the  titration  is  performed  must  not  be  too  much  at  variance 
with  the  temperature  at  which  the  instruments  are  graduated 
and  the  solutions  made.  A  temperature  of  16°  C.  is  one 
which  is  exceedingly  difficult  to  maintain  in  the  warm  months 
of  the  year,  therefore  it  has  been  suggested  to  take  a  higher 
temperature  as  the  standard. 

The  U.S. P.  VIII  recommends  the  employment  of  25°  C. 
(77°  F.)  as  the  standard.  This,  while  better  than  the  lower 
temperature,  is  regarded  by  many  as  being  too  high  and  the 
use  of  20°  C.  (68°  F.)  as  the  standard  is  now  being  very 
favorably  considered,  this  being  nearer  the  average  temperature 
of  the  atmosphere  in  laboratories  throughout  the  year.  AMiat 
ever  temperature  is  adopted,  it  is  at  this  temperature  that 
the  whole  set  of  measuring  instruments  must  be  graduated, 
and  all  titrations  carried  out.  It  would  be  obviously  improper 
to  use  a  burette  graduated  at  16°  C.  and  a  flask  or  cylinder 


VOLUMETRIC  OR  STANDARD  SOLUTIONS  15 

graduated  at  25°  C,  or  to  employ  solutions  at  a  temperature 
which  is  different  from  that  at  which  they  are  made. 

Mil.  The  term  cubic  centimeter  (cc.)  has  been  replaced 
by  the  word  mil  in  the  U.  S.  P.  IX,  and  in  many  other  recent 
books.  The  United  States  Bureau  of  Standards  declared  that 
the  term  cubic  centimeter  was  a  misnomer,  there  being  a  slight 
difference  between  the  thousandth  part  of  a  liter  and  the 
cubic  centimeter,  as  one  liter  was  determined  to  be  equiv- 
alent to  1.000027  cubic  decimeters.  Hence  the  word  mil, 
an  abbreviation  of  milliliter,  was  adopted  and  represents  the 
thousandth  part  of  a  liter. 

The  U.  S.  P.  liter  is  the  volume  occupied  by  996.04  gms. 
of  distilled  water,  weighed  in  air  with  brass  weights  at  a  tem- 
perature of  25°  C.  and  barometric  pressure  of  760  mm. 

To  Titrate  a  substance  means  to  test  it  volumetrically 
for  the  amount  of  pure  substance  it  contains.  The  term  is 
used  in  preference  to  "  tested  "  or  "  analyzed,"  because  these 
terms  may  relate  to  qualitative  examinations  as  well  as 
quantitative,  whereas  titration  applies  only  to  volumetric 
analysis. 

Residual  Titration,  Re-titration,  sometimes  called  Back 
Titration,  consists  in  treating  the  substance  under  examina- 
tion with  standard  solution  in  a  quantity  known  to  be  in 
excess  of  that  actually  required;  the  excess  (or  residue)  is 
then  ascertained  by  residual  titration  with  another  standard 
solution. 

Thus  the  quantity  of  the  first  solution  which  went  into 

combination  is  found. 

N 
Example.     Ammonium  carbonate  is  treated  first  with  — 

H2SO4  in  excess,  and  the  excess  then  found  by  titration  with 

^  KOH. 

I 


16        THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

N 
The  quantity  of  the  —  KOH  used  is  then  deducted  from 

N 
the  quantity  of  —  H2SO4  added,  which  gives  the  quantity  of 

the  latter  which  was  neutrahzed  by  the  ammonium  carbonate. 

Titrations  may  be  carried  out  in  flasks,  beakers,  or  in 
white  porcelain  evaporating  dishes.  Flasks  of  the  Erlenmeyer 
pattern,  see  Fig.  24,  having  a  short  narrow  neck  and  a  broad 
flat  bottom,  are  very  desirable  for  this  purpose;  they  admit 
of  shaking  their  contents  without  danger  of  loss,  and  permit 
ready  observation  of  color  changes.  If  a  flask  is  used  the 
tip  of  the  burette  should  extend  well  into  its  neck  in  order 
to  prevent  any  loss  of  the  reagent.  The  flask  should  be 
rotated  after  each  addition  of  the  reagent,  and  when  the  end 
of  titration  is  near,  any  of  the  solution  adhering  to  the  sides 
of  the  flask  should  be  washed  down  with  distilled  water. 
A  white  porcelain  tile,  or  a  sheet  of  white  paper  placed  under 
the  flask  or  beaker,  aids  materially  in  the  observation  of  the 
color  change. 


CHAPTER  IV 
INDICATORS* 

In  volumetric  analysis  the  substance  to  be  analyzed  in 
the  state  of  solution  is  placed  in  a  beaker  and  the  standard 
solution  is  added  from  a  burette  until  a  certain  reaction  is 
produced.  The  exact  moment  when  a  sufficient  quantity  of 
the  standard  solution  has  been  added  is  known  by  certain 
visible  changes,  which  differ  according  to  the  substance  analyzed 
and  the  standard  solution  used.  When  such  a  visible  change 
occurs  the  "  end-reaction  "  is  reached. 

The  end-reaction  manifests  itself  in  various  ways,  as 
follows : 

1.  Cessation  of  precipitation. 

2.  First  appearance  of  a  precipitate. 

3.  Change  of  color. 

In  some  cases,  however,  the  addition  of  the  standard 
solution  to  the  substance  under  analysis  does  not  produce 
either  a  precipitate  or  a  change  of  color;  in  such  cases  we 
must  resort  to  the  use  of  an  indicator. 

^  An  indicator  is  a  substance  which  is  used  in  volumetric 
analysis,  and  which  indicates  by  change  of  color,  or  some 
other  visible  sign,  the  exact  point  at  which  a  given  reaction 
is  complete. 

Generally  the  indicator  is  added  to  the  substance  under 
examination,  but  in  a  few  cases  it  is  used  alongside,  a  drop 
of  the  substance  being  occasionally  brought  in  contact  with 
a  drop  of  the  indicator. 

*  A  more  detailed  description  of  the  individual  indicators  is  given  in 
the  Appendix.  . 

17 


18        THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

Thus  in  estimating  an  alkali  with  an  acid  volumetric  solu- 
tion the  alkali  is  shown  to  be  completely  neutralized  when 
the  litmus  tincture  which  was  added  becomes  faintly  red  or 
the  phenolphthalein  colorless.  Again,  when  haloid  salts  are 
estimated  with  nitrate  of  silver  solution,  chromate  of  potas- 
sium is  added  as  indicator.  A  white  precipitate  is  produced 
as  long  as  any  halogen  is  present  to  combine  with  the  silver, 
and  when  all  is  precipitated  the  chromate  of  potassium  acts 
upon  the  silver  nitrate,  forming  the  red  silver  chromate,  this 
color  thus  showing  that  all  the  halogen  has  been  precipitated. 

INDICATORS  COMMONLY  USED 

The  principle  indicators  used  are: 

Tincture  of  Litmus,  which  shows  acidity  by  turning  red 
and  alkalinity  by  becoming  blue. 

Phenolphthalein  Solution,  which  is  colorless  in  acid  solu- 
tions and  red  in  alkaline  solutions,  but  is  not  reliable  for 
alkaline  phosphates,  bicarbonates  or  ammonia. 

Methyl-orange  Solution  turns  red  with  acids  and  yellow 
with  alkalies.  It  is  not  affected  by  carbonic  acid,  and  is 
therefore  adapted  for  the  titration  of  alkaline  carbonates. 

Rosolic-acid  Solution  is  yellow  with  acids  and  violet-red 
with  alkalies.     It  is  very  sensitive  to  ammonia. 

Tincture  of  Turmeric  turns  brown  with  alkalies,  and  fhe 
yellow  color  is  restored  by  acids. 

Cochineal  Solution  turns  violet  with  alkalies  and  yellowish 
with  acids.  It  is  used  chiefly  in  the  presence  of  ammonia 
or  alkali  earths. 

Eosin  Solution  is  red  by  transmitted  light,  and  shows  a 
strong  green  fluorescence  by  reflected  light.  Acids  destroy 
this  fluorescence  and  alkalies  restore  it. 

Brazil-wood  Test-solution  turns  purplish-red  with  alkalies 
and  yellow  with  acids. 


INDICATORS  19 

Fluorescein  Test-solution  shows  a  strong  green  fluorescence 
by  reflected  light  in  the  presence  of  the  least  excess  of  an 
alkah. 

Neutral  Potassium-chromate  Test-solution  is  used  in  the 
titration  of  haloid  salts  with  silver-nitrate  solution.  It  indicates 
that  all  the  halogen  has  combined  with  the  silver  by  producing 
a  red-colored  precipitate  (silver  chromate). 

Potassium-ferricyanide  Test-solution  is  used  in  the  estima- 
tion of  ferrous  salts  with  potassium-dichromate  solution.  It 
gives  a  blue  color  to  a  drop  of  the  solution  on  a  white  slab 
as  long  as  any  iron  salt  is  present  which  has  not  been  oxidized 
to  ferric. 


THE   IONIZATION   OR   DISSOCIATION   THEORY 

When  a  soluble  salt  dissolves  in  water,  its  molecules  split 
up  or  dissociate  more  or  less  completely  into  parts  called 
ions.  This  behavior  of  substances,  on  going  into  solution, 
is  known  as  electrolytic  dissociation  or  ionization. 

Ions  are  electrically  charged  atoms  or  groups  of  atoms 
and  are  of  similar  composition  to  the  substances  formed  from 
the  compound  when  an  electric  current  is  passed  through  the 
solution.  The  electro-positive  ions  migrate  to  and  collect 
around  the  negative  pole  (cathode)  and  hence  are  called 
cathions,  while  the  electro-negative  ions  are  called  anions, 
because  they  concentrate  around  the  positive  pole  or  anode. 
The  dissociation  of  a  compound  into  its  ions  when  an  electric 
current  is  passed  through  its  solution,  although  spoken  of 
as  electrolytic  dissociation,  is  really  not  caused  by  the  electric 
current,  since  the  dissociation  into  ions  occurs  at  once 
upon  dissolving  the  substance  in  water  and  without  the  aid 
of  an  electric  current,  the  action  of  the  current  being  the 
transportation  of  the  separated  ions  to  the  poles. 


20        THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

The  dissociation  of  compounds  into  ions  when  dissolved 
in  water  is  illustrated  in  the  following  list: 

Sodium  chlorid  into (Na  + )  and  (CI  — ) 

Potassium  nitrate  into  .^ (K  + )    "    (NO3  - ) 

hydroxidinto (K  +  )    "    (0H-) 

"         acetate  into (K  +  )    "    (C2H3O2-) 

Sulphuric  acid  into *. . . .  (H+)    "    (HSO4-) 

or  (H  +  )    ''    (H  +  )and  (SO4) 

The  extent  of  this  dissociation  depends  upon  the  nature 
of  the  substance  and  the  degree  of 'dilution;  the  greater  the 
dilution  the  more  complete  the  dissociation.  Furthermore, 
strong  acids  and  bases  dissociate  readily,  even  in  compara- 
tively concentrated  solutions,  while  the  weaker  acids  and  bases 
are  more  or  less  undissociated  when  dissolved,  i.e.,  they  are 
not  readily  split  up  into  Has.  Their  salts,  however,  are 
immediately  and  completely  ionized.  Therefore,  upon  neu- 
tralizing a  weak  acid  or  base,  an  ionizable  salt  is  formed. 
According  to  the  theory  of  Arrhenius,  the  reactions  of  analytical 
chemistry  are  chiefly  reactions  between  ions  and  not  between 
atoms. 

Strong  acids,  bases  and  salts  exist  in  solution,  not  as 
molecules  but  chiefly  in  the  form  of  ions.  The  formation 
of  silxer  chlorid  by  the  reaction  between  silver  nitrate  and 
sodium  chlorid  takes  place  according  to  the  following  equation: 

Ag/NOsAq.  +Na/ClAq.  =  AgCl(  solid)  +Na/N03Aq. 

The  state  of  dissociation  being  denoted  by  the  vertical 

line  between  the  ions  of  the  molecules. 

+  -f-     - 

This  theory  also  explains  why  K/CIO3  with  Ag/NOs  does 

"not  form  AgCl,  in  that  the  reaction  involves  the  ion  CiOs 
and  not  the  atom  CL 


INDICATORS  21 

Theories  of  Indicators.  In  connection  with  the  use  of 
indicators  in  neutralization  analyses,  the  question  as  to  the 
cause  of  the  color  changes  is  one  of  considerable  interest. 

Two  distinct  views  are  held.  Of  these  the  Ionization 
Theory  of  Ostwald  has  received  almost  universal  acceptance. 
According  to  this  theory  the  color  changes  are  ascribed  to  a 
change  in  the  indicator  from  a  molecular  to  an  ionic  condition. 
As  exemplified  in  the  case  of  phenolphthalein  the  colorless 
molecule 

OCOC6H4C-(C6H40H)2       ....     (I) 

I I 

is  dissociated  into  the  red  negative  ion 

OCOC6H4C.(C6H40H)C6H40.     .     .     .     (II) 

In  the  other  and  less  kno^  view  on  this  subject  (the 
Chromophoric  Theory),^  the  sensitiveness  of  the  indicators 
and  its  color  changes  is  ascribed  to  a  change  in  the  consti- 
tution of  the  indicator,  involving  a  cl%omophoric  group,  under 
the  influence  of  hydrogen  and  hydroxyl  ions.  According  to 
this  view  the  color  change  is  due  (in  the  case  of  phenol- 
phthalein) to  a  change  of  constitution  from  the  colorless  lactoid 
(I)  with  no  chromophoric  group,  to  the  colored  quinoid  (III) 
with  a  quinone  chromophore. 

(NaOOC-C6H4)(HOC6H4)C:C6H4:0,  .     .     (Ill) 

and  that  the  ionization  of  the  sodium  salt  is  merely  a  coin- 
cidence and  not  the  cause  of  the  color  change. 

Whichever  of  these  views  is  the  correct  one,  remains  for 
future  investigations  to  prove.  That  of  Ostwald,  being  most 
generally  accepted  at  the  present  time,  is  treated  more  fully 
below. 

♦  Sec  Julius  St3>glitz,  Jour.  Am.  Chem.  Soc,  XXV,  in 2  (1903). 


22        THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

The  Ionization  Theory  of  Indicators.*  The  indicators  used 
in  alkalimetry  and  acidimetry  are  compounds  of  feeble  acid 
or  basic  character,  and  hence  not  prone  to  dissociation  in 
solution,  but  when  neutralized  the  salt  formed  ionizes  the 
instant  of  its  formation;  the  ions  so  liberated  give  rise  to 
colors  which  differ  from  those  of  the  original  compounds. 

Any  feeble  acid  or  base  may  be  utilized  as  an  indicator 
if  its  ions  have  a  color  different  from  that  of  the  un-ionized 
compound.  Strong  acids  or  bases  are  not  suited  as  indicators, 
because  they  ionize  while  in  a  free  state  on  dilution,  and  thus 
give  no  color  when  neutralized. 

A  solution  in  which  H  ions  predominate  has  an  acid  reac- 
tion, while  one  in  which  OH  ions  predominate  reacts  alkaline. 

Phenolphthalein  is  a  feebly  acid  indicator,  and  in  its 
undissociated  state  is  colorless.  It  does  not  dissociate  readily 
unless  neutralized,  but  when  sodium  hydroxid  is  added  to 
its  solution,  a  sodium  salt  of  phenolphthalein  is  formed  which 
immediately  ionizes  and  the  ions  liberated  impart  to  the  solution 
a  brilliant  red  color.  If  now  some  acid  is  added  the  sodium 
salt  is  decomposed  and  the  acid  phenolphthalein  again  set 
free,  and  the  solution  becomes  colorless. 

If  a  few  drops  of  phenolphthalein  solution  be  added  to 
an  acid  solution,  ionization  of  the  former  is  prevented  by 
the  presence  of  the  stronger  acid;  if  now  some  sodium  hydroxid 
solution  is  added,  the  OH  ions  of  the  latter  unite  with  the 
H  ions  of  the  acid,  and  when  the  acid  is  completely  neutralized 
the  first  drop  of  excess  of  alkali  unites  with  the  phenolphtha- 
lein, forming  a  salt  which  immediately  ionizes  and  produces 
the  characteristic  red  color  which  shows  the  end  of  the  reaction. 

In  the  titration  of   a  feeble  acid    the  end-point  is   often 

*  See  Ostwald's  'Lehrbuch  der  AUgemeinen  Chemie,"  1891,  and  "Scientific 
Foundations  of  Analytical  Chemistry,"  1900;  also  Walker's  "Introduction  to 
Physical  Chemistry,"  1899. 


INDICATORS  23 

indistinct  and  is  lacking  in  sharpness;  this  is  because  the 
indicator  used  has  a  greater  tendency  to  ionize  than  the  acid 
itself.  In  this  case  the  H  ions  present  just  before  the  com- 
pletion of  the  reaction  are  not  in  sufficient  amount  to  fully 
retard  the  ionization  of  the  indicator,  and  hence  the  latter 
dissociates  partly  before  neutralization  is  complete  and  gives 
rise  to  an  indefinite  end-reaction.  Therefore  it  is  necessary 
when  titrating  a  feeble  acid  that  an  indicator  should  be  selected 
whose  alkali  salt  ionizes  with  the  production  of  a  distinct 
color  change,  and  whose  tendency  to  ionize  is  less  than  that 
of  the  acid.  Phenolphthalein  is  a  suitable  indicator  in  this 
case,  provided  a  strong  alkali  be  used  for  titrating. 

Fixed  alkalies  readily  yield  ionizable  salts  with  phenol- 
phthalein, but  ammonia  does  not.  The  latter  being  too  weak 
a  base  to  yield  with  so  feeble  an  acid,  a  salt  which  can  with- 
stand the  hydrolytic  action  of  the  water  in  dilute  solutions, 
and  as  a  consequence  a  larger  excess  of  the  ammonia  must 
be  used  to  overcome  this.  Thus  is  accounted  for  the  imperfect 
color  change  of  phenolphthalein  when  ammonia  or  its  salts 
are  present  and  why  the  color  becomes  visible  only  after  a 
large  excess  of  the  alkali  is  added. 

Paranitrophenol  is  also  an  acid  indicator;  it  exists  in 
solution  in  the  form  of  undissociated  colorless  molecules,  yet 
its  electro-negative  ion  is  intensely  yellow  in  color.  This  com- 
pound has  a  slight  tendency  to  dissociate  in  dilute  solutions, 
but  the  presence  of  a  trace  of  a  stronger  acid  will  overcome 
this  tendency  and  the  solution  remains  colorless.  .If  an  alkali 
is,  however,  added,  a  salt  of  paranitrophenol  is  formed  which 
immediately  ionizes  and  exhibits  the  intense  yellow  color  of 
its  liberated  ion. 

Other  indicators  exhibit  a  color  in  both  the  ionized  and 
the  non-ionized  state,  but  the  colors  in  both  conditions  are 
different,  as  in  the  case  of  litmus,  lacmoid  and  methyl  orange. 


24        THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

Methyl  orange  is  both  an  acid  and  a  base  and  will  form 
ionizable  salts  with  either  acids  or  alkalies;  its  indicator 
characteristics  are,  however,  due  essentially  to  its  basic  char- 
acter. 

The  salt  which  methyl  orange  forms  with  acids  dissociates 
into  red  ions;  this,  upon  the  addition  of  an  alkali,  returns  to 
its  undissociated  state,  which  is  yellow.  Because  of  its  weak 
basic  character  its  compound  with  acids  is  readily  decomposed 
by  alkaUes,  but  it  takes  a  strong  acid  or  a  relatively  large 
quantity  of  a  feeble  acid  to  dissociate  it  in  its  non-ionized 
state,  hence  this  indicator  is  very  sensitive  to  alkalies,  and 
much  less  so  to  acids. 

With  reference  to  the  acid  character  of  the  indicator  the 
explanation  of  its  action  is  that  the  non-ionized  indicator  is 
red,  while  its  ion  is  yellow.  Acids  lessen  its  ionization  and 
produce  a  red  color,  while  alkalies  produce  a  highly  ionizable 
salt  and  hence  a  yellow  color.  When  a  weak  but  slightly 
ionizable  acid  is  added  to  the  methyl  orange  solution,  the  H* 
ions  of  the  acid  given  up  in  excess  of  the  amount  required  for 
neutralization  are  not  sufficient  to  yield  enough  of  a  non- 
ionizable  salt  to  produce  a  decided  red  color,  hence  a  large 
quantity  of  such  a  weak  acid  is  required  to  give  an  acid 
indication.  This  would  explain  why  methyl  orange  is  not 
suitable  as  an  indicator  for  weak  acids,  and  why  it  is  very 
sensitive  to  alkalies. 

Litmus  is  an  acid  indicator  which  sUghtly  dissociates  in 
solution.  Its  non-ionized  molecules  are  red,  but  its  negative 
ions  are  blue.  If  it  is  added  to  an  alkali,  a  salt  is  formed 
which  at  once  ionizes  and  gives  a  blue  color.  If  added  to 
an  acid,  ionization  is  prevented  and  the  red  color  of  the  non- 
ionized  molecules  appears. 

From  the  above  explanations  it  will  be  seen  that  indicators 
cannot  be  indiscriminately  used,  and  that  no  one  indicator 


INDICATORS  25 

will  be  suitable  for  every  titration.  Hence  the  indicator  must 
be  selected  to  suit  each  case.  This  selection  is  facilitated  by- 
reference  to  the  classification  of  indicators,  according  to  F. 
Glaser,  Ztchr.  f.  a.  Chem.,  1899,  273  +  . 

Group  I.  Indicators  Forming  Fairly  Stable  Salts.  The 
members  of  this  group  comprise  such  indicators  as  are  (i)  of 
a  strong  acid  character  and  which  react  readily  with  weak 
bases,  or  (2)  of  a  feeble  basic  character  and  which  require 
a  strong  acid  to  form  a  stable  salt.  Hence  they  will  be  found 
to  be  very  sensitive  to  alkalies,  and  are  useful  in  the  titration 
of  weak  bases,  as  ammonia  and  the  amine  bases,  as  well  as 
strong  bases  and  acids.  The  indicators  of  this  group  are 
the  following,  arranged  in  the  order  of  their  sensitiveness 
towards  alkalies: 

(i)  lodeosin,  Resazurin;  (2)  Tropaeolin  OO,  Luteol;  (3) 
Methyl  and  Ethyl  Orange;  (4)  Congo  Red;  (5)  Cochineal; 
(6)  Lacmoid. 

Group  II.  Indicators  Possessing  Faint  Acid  Properties  and 
Yielding  Salts  which  are  Very  Unstable.  These  are  readily 
decomposed  by  relatively  feeble  acids,  and  are  in  consequence 
very  sensitive  towards  acids,  slightly  so  towards  alkalies. 

They  are:  (i)  Rosolic  acid;  (2)  Curcuma;  (3)  Phenol- 
phthalein,  Flavescin;   (4)  Alpha-naphtholbenzein. 

Grjup  III.  Indicators  Occupying  a  Place  Midway  between 
the  Other  Two  Groups.  They  are  somewhat  stronger  acids 
than  those  of  Group  II,  but  feebler  than  those  of  Group  I. 

They  are  fairly  sensitive  towards  both  acids  and  alkalies, 
but  are  more  sensitive  towards  acids  than  those  of  Group  I, 
and  less  so  towards  alkalies.     They  are: 

(i)  Fluorescein,  Phenacetolin;  (2)  Haematoxylin,  Gallein, 
Alizarin;    (3)  Litmus;    (4)  Paranitrophenol. 

This  division  of  indicators  into  groups,  as  above,  facilitates 
the  selection  of  an  indicator  suitable  for  the  work  in  hand. 


26        THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

For  instance,  for  titrating  weak  acids,  a  glance  at  the  groups 
will  show  that  the  members  of  Group  II  are  best  adapted 
for  this  purpose.  Again,  weak  bases  will  be  best  titrated  by 
the  indicators  of  Group  I.  Strong  acids  or  bases  may  be 
titrated  by  means  of  any  of  the  indicators. 

The  quantity  of  indicator  taken  in  a  titration  is  a  matter 
of  considerable  moment.  The  smallest  quantity  which  will 
produce  a  distinct  color  should  be  taken,  but  it  is  equally 
important  that  the  quantity  be  not  too  small  for  the  volume 
of  liquid;  for  in  high  dilutions  the  hydrolytic  action  of  water 
asserts  itself,  and  intermediate  tints  will  result,  which  interfere 
with  the  sharpness  of  the  end  color. 

If  too  much  of  the  indicator  is  used,  the  sensitiveness  is 

lessened,  because  acid  or  alkali  must  be  added  to  convert  the 

indicator  into  a  salt,  or  when  formed  to  decompose  it;  i.e., 

a  minimum  of  excess  of  the  titrating  fluid  would  react  with 

a  small  portion  only  of  the  indicator  and  intermediate  tints 

would  be  produced,  until  sufficient  of  the  titrating  solution 

has  been  added  to  neutralize  all  of  the  indicator  present.     This 

is  especially  true  when  using  centinormal  solutions.     20  drops 

of  litmus  added  to  10  mils  of  water  require  from  10  to  14  drops 

N 

of acid  or  alkaU  solution  to  produce  a  change  of  color. 

100 

Thus  the  indicator  itself  takes  up  some  of  the  standard 
solution,  and  hence  the  necessity  for  using  as  small  a  quantity 
of  the  indicator  as  possible;  usually  from  j  to  j  drops  of  the 
indicator  may  be  taken  to  each  jo  or  100  cc.  of  the  fluid  titrated. 

The  degree  of  dilution  of  the  substance  titrated  is  also  a 
matter  of  considerable  moment.  In  very  concentrated  solu- 
tions ionization  does  not  so  readily  occur,  while  too  great  a 
dilution  diminishes  the  reactive  ability  of  the  ions  because 
of  their  greater  separation,  and  also  because  of  the  hydro- 
lytic dissociation  of  water  itself  into  H"  and  OH  ions  which 


INDICATORS  27 

react  acid  and  alkaline  respectively,  and  which  brings  about 
a  premature  dissociation  of  the  indicator. 

The  Requirements  of  a  Good  Indicator,  according  to  H. 
A.  Cripps,  are: 

I.  The  end-reaction  should  be  marked  by  a  prominent 
change  of  color. 

II.  The  smallest  possible  quantity  of  the  reagent  should 
be  required  to  effect  this  change. 

III.  High  tintorial  power,  which  of  itself  assists  in  the 
fulfilment  of  the  second  requirement,  less  of  the  indicator 
being  required. 

IV.  The  change  of  color  should  not  be  affected  by  the 
impurities  commonly  present  in  the  substance  under  examina- 
tion, nor  by  the  products  of  the  reaction. 

In  addition  to  these  requirements  it  is  a  distinct  advantage 
if  the  color  reaction  is  equally  decided  in  alcoholic  as  in  aque- 
ous liquids. 

A   GUIDE    FOR   THE   SELECTION   OF   INDICATORS 

For  Hydroxids  and  Carbonates 

Indicators  not  affected  by  COj  Indicators  affected  by  CO, 

(Cold  Titrations)  (Hot  Titrations) 

Methyl  Orange,  Gallein,  Phena-  Phenolphthalein        (useless-      in 

cetolin,      Congo     Red,     lodeosin,      presence    of    NH3    or    its    salts), 
Cochineal.  Lacmoid,  Rosolic  Acid,  Resazurin. 


For  Ammonia  {NH3)  For  Ammonium  Carbonate 

Rosolic   Acid,    Methyl    Orange,  Same  indicators  as  for  ammonia, 

Congo  Red,  Litmus,  Gallein.  also     Phenacetalin     and     Phenol- 

phthalein. 


28        THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

For  Inorganic  Acids  For  Organic  Acids 

if  2^0 4,  HCl,  HNO3.  Phenolphthalein      (all)      Rosolic 

Phenolphthalein,  Litmus,  Rosolic      Acid     (except     acetic,     citric    and 
Acid,  Methyl  Orange.  tartaric),  Gallein. 

Phenolphthalein  (neutralized  to 
NazHPO^). 

Methyl  Orange  and  Cochineal 
(each  neutralized  to  NaHgPOJ. 

Rosolic  Acid  and  Methyl  Orange. 

Phenolphthalein  (after  addition 
of  glycerin),  Litmus  and  Turmeric 
paper. 


CHAPTER  V 


APPARA'rUS  USED  IN  VOLUMETRIC  ANALYSES 


The  Burette  is  a  graduated  glass  tube  wHich  holds  from 
25  to  100  mils  and  is  graduated  in   fifths  or  tenths  of  a  mil. 

and  provided  at  the  lower  end  with 
a  rubber  tube  and  pinch-cock.  The 
use  of  this  instrument  is  to  accurate- 
ly measure  quantities  of  standard 
solutions  used  in  an  analysis.  It  is 
in  an  upright  position  when  in  use, 
and  the  flow  of  the  solution  can  be 
regulated  so  as  to  run  out  in  a 
stream  or  flow  in  drops  by  pressing 
the  pinch-cock  between  the  thumb 
and  forefinger.  The  quantity  of 
solution  used  can  be  read  from  the 
graduation  on  the  outside  of  the 
tube.  This  is  the  simplest  and  most 
common  form  of  burette,  and  is 
known  sls  Mohr^s  (Fig.  i). 

The  use  of  the  pinch-cock  in 
Mohr's  burette  may  be  dispensed 
with  by  introducing  into  the  rubber 
tube  a  small  piece  of  glass  rod, 
which  must  not  fit  too  tightly.  By 
firmly  squeezing  the  rubber  tube  surrounding  the  glass  rod  a 
small  canal  is  opened,  through  which  the  liquid  escapes.     A 

29 


Fig. 


30        THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 


48- 


very  delicate  action  can  in  this  way  be  obtained,  and  the  flow 
of  the  liquid  is  completely  under  the  control  of  the  operator. 
(See  Fig.  2.) 

The  greatest  drawback  to  this  burette  is  that  it  cannot 
be  used  for  permanganate  or  other  solu- 
tions that  act  upon  the  rubber. 

This  defect  can  be  overcome  by  the  use 
of  a  burette  having  a  glass  stop-cock  in 
place  of  the  rubber  tubing  and  pinch- 
cock.  This  form  has  the  additional  advan- 
tage of  being  capable  of  delivering  the  solution 
in  drops  while  both  hands  of  the  operator  are 
disengaged  (Fig.  3). 

Another  good  arrangement  is  that  in 
which  the  tap  is  placed  in  an  oblique  position, 
so  that  it  will    not  easily  drop   out  of    place 

(Fig.  4)- 

These  glass  stop-cock  burettes  should  be 
emptied  and  washed  immedi- 
ately after  use,  especially  if  soda 
or  potassa  solution  has  been 
used;  for  these  act  upon  the 
glass  and  often  cause  the  stop- 
per to  stick  so  firmly  that  it 
cannot  be  turned  or  removed 
without  danger  of  breaking 
the  instrument. 

The  most  satisfactory  form 
of  glass  stop-cock  is  that  shown 
in  Fig.  5. 

When  a  number  of  estima- 
tions are  to  be  made  in  which  the  same  volumetric  solution  is 
employed,  the  arrangement  shown  in  Fig.  6  is  very  serviceable. 


50- 


FiG.  3.. 


Fig.  4- 


APPARATUS  USED  IN  VOLUMETRIC  ANALYSIS        31 

A  T-shaped  glass  tube  is  inserted  between  the  lower  end 
of  the  burette  and  the  pinch-cock  and  connected  by  a  rub- 
ber tube  with  a  reservoir  containing  the  volumetric  solution. 

The  tube  which  communicates  with  the  reservoir  is  provided 
with  a  pinch-cock,  which,  when  open,  allows  the  solution  to 
flow  into  and  fill  the  burette  in  so   gradual  a   manner  that 


Fig.  5. 


Fig.  6. 


no  bubbles  are  formed.     The  burette  is  emptied  in  the  usual 
manner. 

E.  ^  A.  Automatic  Burette  (Fig.  7).  This  is  used  for 
the  same  purpose  as  the  foregoing.  It  is  provided  with  a 
side  tube  for  connection  with  reservoir,  and  has  an  overflow 
cup  which  prevents  its  being  filled  to  above  the  zero  mark. 
The  three-way  stop-cock  is  so  arranged  that  if  turned  one 


32        THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

way  the  inlet  is  opened  and  the  liquid  from  the  reservoir  flows 
into  and  fills  the  burette.  If  turned  the  other  way  the  inlet 
is  closed  and   the  outlet  is  opened  and    the  burette  may  be 


Fig.  7. 


Fig.  8. 


emptied.  If  the  handle  of  the  stop-cock  is  turned  half-way 
round,  both  openings  are  closed. 

There  are  many  other  forms  of  automatic  burettes. 

When  working  with  solutions  which  are  readily  altered 
by  contact  with  air,  as  for  example,  stannous  chlorid,  potas- 


APPARATUS  USED    IN   VOLUMETRIC  ANALYSIS     33 


slum,  sodium,  or  barium  hydroxid  or   ammonia,   an  arrange- 
ment like  that  depicted  in  Fig.  8  is  very  serviceable.     In  this 

the  upper  end  of  the  burette  is 
connected  with  the  reservoir  by 
means  of  a  rubber  tube,  thus 
making  an  air-tight  combination 
between  the  burette  and  the  reser- 
voir. Its  utility  may  be  further 
enhanced  by  providing  the  reser- 
voir with  a  soda-lime  tube  or  some 
other  suitable  absorption  tube. 

Another  form  of  apparatus  is 
shown  in  Fig.  9.  In  this  both 
the  burette  and  the  reservoir  are 
provided  with  tubes  containing 
soda-lime  to  insure  a  protection 
against  the  admission  of  CO2 
and  moisture  from  the  air. 

Pinch-cocks  used  with  Mohr's 
burettes  are  of  various  kinds. 
See  Figs.  10,  11  and  12. 

The  screw  pinch-cock,*  Fig. 
12,  is  a  very  useful  device;  it 
may  be  used  like  the  ordinary 
pinch-cock  by  pressure  with  the 
fingers  upon  a-a,  when  a  rapid 
flow  is  desired,  or  the  nut-screw 
ih)  may  be  so  adjusted  as  to 
allow  a  slower  flow  or  to  deliver 
the  solution  in  drops,  thus  giving  the  operator  the  freedom 
of  both  hands  for  other  work. 

Burette  supports  are  of  various  forms.     One  of  the  best 

*  W.  V.  Her^endorf. 


Fig.  9. 


34        THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

for  one  or  two  burettes  is  shown  in  Fig.  13.  It  is  made  of 
iron,  can  stand  firmly  upon  an  uneven  surface,  and  does  not 
easily  tip  over.  The  burettes  are  fastened  to  it  by  means 
of  clamps,  illustrated  in  Figs.  14  and  15. 

A  revolving  burette-holder  for  eight  burettes  is  shown  in 


Fig.  II. 


Fig.  16.  Burrette-supports  are  also  made  wdth  white  porcelain 
base,  which  enables  the  operator  more  readily  to  see  the  change 
of  color  in  the  liquid  titrated. 

Pipettes  are  of  two  kinds — those  which  are  marked  to 
deliver  one  quantity  only,  and  those  which  are  graduated  on 
the  stem  like  burettes.  Their  use  is  to  measure  out  portions 
of  solutions  with  exactness. 


APPARATUS  USED  IN  VOLUMEiC  TRANALYSIS       35 


Pipettes  are  filled  by  applying  the  mouth  to  the  upper  end 
and  sucking  the  liquid  up  to  the  mark,  then,  by  closing  the 
upper  opening  with  the  forefinger,  the  liquid  is  prevented 
from  running  out,  but  may  be  delivered  in  drops  or  allowed 
to  run  out  to  any  mark  by  lessening  the  pressure  of  the 
finger  over  the  opening. 

In  using  the  pipettes  of  the  first  class  (Fig.  17)  the  finger 


Fig.  14. 


Fig.  13. 

is  raised  and  the  instrument  allowed  to  empty  itself  entirely. 
A  drop  or  two,  however,  usually  remains  in  the  lower  portion 
of  the  instrument,  which  may  be  blown  out,  though  this  is  not 
considered  good  practice.  By  inclining  the  pipette  and  placing 
the  point  against  the  side  of  the  vessel  which  is  to  receive  the 
liquid,  the  instrument  may  be  emptied  more  satisfactorily. 

Pipettes  of  the  second  class  (Fig.  18)  are  never  emptied 
completely  when  in  use.  The  flow  of  the  liquid  is  regulated 
by  the  pressure  of  the  finger  over  the  upper  opening,  and 
stopped  at  the  desired  point. 


36       THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

A  very  convenient  form  of  pipette  is  one  which  has  attached 
to  its  upper  end  a  piece  of  rubber  tubing,  into  which  a  short 
piece    of    glass    rod    has   been    inserted.      By  squeezing  the 


Fig.  i6. 

rubber  surrounding  the  glass  rod  firmly  between  the  fingers,  a 
canal  is  opened  and  the  liquid  can  be  drawn  up  into  the 
pipette  by  suction  with  the  lips  and  run  out  again.  By  re- 
moving the  pressure  the  canal  closes  and  the  flow  of  the 
liquid  is  stopped  at  any  point  (Fig.  19). 


APPARATUS  USED  IN  VOLUMETRIC  ANALYSIS       37 

The  Nipple  Pipette  is  very  convenient  for  measuring  small 
quantities  of  liquids,  such  as  i  or  2  mils  (Fig.  20). 

When  a  volatile  or  highly .  poisonous  solution  is  to  be 
measured  it  is   not  advisable  to  suck  it  up  with  the  mouth. 


50  mils 


IQ  mils 


Fig.  17. 


Fig.  18. 


The  pipette  in  this  case  is  filled  by  dipping  it  into  the  liquid 
contained  in  a  long,  narrow  vessel,  until  the  liquid  reaches 
the  proper  mark  on  the  pipette,  then  closing  the  upper  opening 
and  withdrawing.  .When  this  is  done  the  liquid  which 
adheres  to  the  outside  of  the  pipette  should  be  dried  off 
before  the  measured  liquid  is  delivered. 

A  French  firm  has  introduced  pipettes  provided  with  suction 


38 


THE  ESSENTIALS  OF  \  OLUMETRIC  ANALYSIS 


pumps,  shown  in  various  forms  by  Fig.  21,  which  possess 
the  advantage  over  the  ordinary  forms  provided  with  a  com- 
pressible rubber  bulb,  that  the  liquid  can  with  perfect  ease 
be  drawn  up  to  the  desired  point  on  the  scale,  and  with 
absolute  accuracy  maintained  at  the  same  height  as  long  as 
may  be  desired. 

The  Measuring-flask  is  a  vessel  made  of  thin  glass  having 


Fig.  20, 


Fig.  21. 


a  narrow  neck,  and  so  constructed  as  to  hold  a  definite  amount 
of  liquid  when  filled  up  to  the  mark  on  the  neck.  These 
flasks  are  of  various  sizes,  holding  100,  250,  500  1000  mils, 
etc.,  but  are  generally  called  "  Liter  Flasks^     (Fig.  22.) 

Liter  flasks  are  used  for  making  volumetric  solutions. 

Those  which  have  the  mark  below  the  middle  of  the  neck 
are  to  be  preferred,  because  the  contents  can  be  more  easily 
shaken. 


APPARATUS  USED  IN  VOLUMETRIC  ANALYSIS        39 


Liter  flasks  are  sometimes  made  with  two  marks  on  the 
neck  very  near  together;  the  lower  one  is  the  Hter  mark.  If 
the  flask  is  required  to  deliver  a  hter,  it  must  be  filled  to  the 
upper  mark,  the  difference    between  the  two  measures  being 


Fig.  22. 


Fig.  23. 


Fig.  24. 


the  equivalent  of  the  liquid  which  remains  in  the  flask  adhering 
to  the  sides. 

The  Test-mixer,  or  Graduated  Cylinder  (Fig.  23)  is  for 
measuring  and  mixing  smaller  quantities  of  solutions.  They 
are  made  of  different  sizes,  holding  100,  250,  500  and  1000 
mils,  and  graduated  in  fifths  or  tenths  of  a  mil. 

Titration  Flasks.  Titrations  may  be  carried  out  in  flasks 
of  any  usual  shape,  or  in  beakers,  or  evaporating  dishes,  but 
the  flask  illustrated  in  Fig.  24  is  to  be  preferred. 


CHAPTER  VI 
ON  THE  USE  OF  APPARATUS 

It  is  important  that  all  apparatus  used  in  volumetric 
analysis  be  perfectly  clean.  Even  new  apparatus  should  be 
cleansed  by  passing  dilute  hydrochloric  acid  through  them 
and  then  rinsing  with  distilled  water. 

If  the  burette,  pipette,  or  other  instrument  is  even  slightly 
greasy,  the  liquid  will  not  flow  smoothly,  and  drops  of  the 
liquid"  will  remain  adhering  to  the  sides,  thus  leading  to 
inaccurate  results. 

Greasiness  may  be  removed  with  dilute  soda  solution. 
If  this  fails  the  instrument  should  be  allowed  to  remain  for 
some  little  time  in  a  solution  containing  sulphuric  acid  and 
potassium  dichromate,  which  will  radically  remove  all  traces 
of  grease. 

The  burette  or  other  measuring  instruments  should  never 
be  filled  with  volumetric  solution  without  first  rinsing,  even 
if  the  burette  be  perfectly  dry. 

It  is  well  to  wash  the  inside  of  the  instrument  with  two 
or  three  small  portions  of  the  solution  with  which  it  is  to  be 
filled. 

The  burette  may  be  filled  with  the  aid  of  a  funnel,  the 
stem  of  which  should  be  placed  against  the  inner  wall  of  the 
burette,  so  that  the  solution  will  flow  down  the  side  and  thus 
prevent  the  formation  of  bubbles. 

The  burette  should  be  filled  to  above  the  zero  mark,  and 
the  air-bubbles,  if  there  are  any,  removed  by  gently  tapping 
with  the  finger. 

40 


ON  THE  USE  OF  APPARATUS  41 

A  portion  of  the  liquid  should  then  be  allowed  to  run  out 
in  a  stream  so  that  no  air-bubbles  remain  in  the  lower  part 
of  the  burette.  In  the  glass  tap  burette  it  can  be  easily  seen 
if  any  air  is  present,  but  in  the  pinch-cock  burette  it  is  some- 
times necessary  to  take  hold  of  the  rubber  tube  between  the 
thumb  and  forefinger  and  gently  stroke  upward.  Or  the 
glass  nip  at  the  lower  end  of  the  burette  may  be  pointed  upward, 
and  the  pinch-cock  opened  wide  so  that  a  stream  of  the  liquid 
will  pass  through  and  force  out  any  air  that  may  be  inclosed. 

If  the  titration  is  to  be  conducted  at  a  high  temperature, 
as  in  the  estimation  of  carbonates,  when  litmus  is  used  as 
the  indicator,  or  in  the  estimation  of  sugar  by  copper  solution, 
a  long  rubber  tube  should  be  attached  to  the  lower  end  of 
the  burette.  The  boiling  can  then  be  done  at  a  little  distance, 
and  the  expansion  of  the  liquid  in  the  burette  avoided.  The 
pinch-cock  is  fixed  about  midway  on  the  tube. 

Hart  calls  attention  to  the  fact  that  if  the  fluid  in  a  burette 
or  pipette  be  run  out  rapidly  at  one  time  and  slowly  at  another, 
different  amounts  of  fluid  are  obtained. 

This  is  due  to  the  adhesion  of  the  fluid  to  the  inner  sides 
of  the  instrument,  and  reading  before  it  has  settled  down. 
It  is  therefore  advisable  always  to  deliver  burettes  slowly, 
as  more  constant  results  are  then  obtained. 

Solutions  which  are  measured  by  means  of  pipettes  should 
be  dilute,  since  concentrated  solutions  adhere  to  glass  with 
different  degrees  of  tenacity,  and  hence  the  amount  of  fluid 
delivered  is  slightly  less  than  that  measured. 

The  temperature  of  the  solutions  measured  should  be 
taken  into  account,  since  all  liquids  are  affected  by  change 
of  temperature,  expanding  and  contracting  as  the  tempera- 
ture is  increased  or  reduced. 

This  change  of  volume  in  the  case  of  standard  solutions 
does  not  exactly  correspond  to  that  in  pure  water;  in  fact. 


42        THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

some  of  them  differ  widely.  The  correction  of  the  volume 
of  a  standard  solution  for  the  temperature  by  the  expansion 
coefficient  of  water  is  not  entirely  satisfactory,  but  in  the  case 
of  very  dilute  solutions  this  may  be  done. 

Casamajor  (C.  N.,  xxxv,  i6o)  gives  the  following  figures 
showing  the  relative  contraction  and  expansion  of  water  below 
and  above  15°  C: 

Degrees  C.  Degrees  C. 

8  —  .000590  17  +  .000305 

9— .000550  18 +  .000473 

10— .000492  19 +  .000652 

1 1  —  .000420  20  +  .00084 1 

12— .000334  21 +.001039 

13— .000236  22 +  .001246 

14— .000124  23 +.001462 

15  — normal  24 +  .00 1686 

16  +  .000147  25  +  .001919 

By  means  of  these  numbers  it  is  easy  to  calculate  the  volume 
of  liquid  at  15°  C.  corresponding  to  any  volume  observed 
at  any  temperature  between  8°  C.  and  25°  C.  If  25  cc.  of 
solution  had  been  used  at  20°  C,  the  table  shows  that  i  cc. 
of  water  passing  from  15°  to  20°  is  increased  to  i. 000841  cc. 
Therefore,  by  dividing  25  cc.  by  1.000841,  the  quotient,  24.97 
cc.  is  obtained,  which  represents  the  volume  at  15°  C.  corre- 
sponding to  25  cc.  at  20°  C. 

These  corrections  are  of  value  only  for  very  dilute  solutions 
and  for  water,  but  useless  for  concentrated  solutions.  Slight 
variations  of  atmospheric  pressure  may  be  disregarded. 

ON  THE  READING  OF  INSTRUMENTS 

In  narrow  vessels  the  surface  of  liquids  is  never  level. 
This  is  owing  to  the  capillary  attraction  exerted  by  the  sides 
of  the  vessel  upon  the  liquid,  drawing  the  edge  up  and  forming 


ON  THE  USE  OF  APPARATUS 


43 


a  saucerlike  concavity  (Fig.  25).  All  liquids  present  this 
concave  surface  except  mercury,  the  surface  of  which  is  convex. 

This  behavior  of  liquids  makes  it  difficult  to  find  a  distinct 
level,  and  in  reading  the  measure  either  the  upper  meniscus 
(a)  or  the  lower  meniscus  (b)  may  be  used  (Fig.  26). 

The  most  satisfactory  results  are  obtained  if  the  lowest 
point  of  the  curve   (b)  is  used,  especially  with  light-colored 


Fig.  25. 


Fig.  26. 


Fig.  27. 


solutions.  But  if  dark-colored  or  opaque  solutions  are  measured 
it  is  necessary  to  use  the  upper  meniscus  (a)  for  reading. 

In  all  cases  the  eye  should  be  brought  on  a  level  with  the 
surface  of  the  liquid  in  reading  the  graduation. 

The  eye  is  very  much  assisted  by  using  a  small  card, 
the  lower  half  of  which  is  black  and  the  upper  half  white, 
This  card  is  held  behind  the  burette,  the  dividing  line  between 
white  and  black  being  about  an  eighth  of  an  inch  below  the 
surface  of  the  liquid.  The  eye  is  then  brought  on  a  level 
with  it,  and  the  lower  meniscus  can  be  distinctly  seen  as  a 
sharply  defined  black  line  against  the  white  background  (Fig. 
27). 


44 


THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 


Erdmann's  Float,  Fig.  28,  is  an  elongated  glass  bulb,  which 
is  weighted  at  its  lower  end  with  mercury,  to  keep  it  in  an 
upright  position  when  floating.     It  is  of  such  diameter  that 


Fig.  28. 


Fig.  29. 


Fig.  30. 


it  will  slide  easily  up  and  down  inside  of  a  burette.  There 
is  a  ring  at  the  top  by  which  it  can  be  lifted  in  or  out  by 
means  of  a  bent  wire.  Around  its  center  a  line  is  marked. 
At  this  line  instead  of  at  the  meniscus  the  reading  is 
taken. 


ON  THE  USE  OF  APPARATUS 


45 


These  floats  are  sometimes  provided  with  a  thermometer, 
and  they  then  register  the  temperature  as  well  as  the  volume. 
Others  are  provided  with  projecting  points  along  the  sides, 


Fig.  31. 


Fig. 


the  object  of  which  is  to  prevent  it  from  adhering  to  the  walls 
of  the  burette.     (See  Fig.  29.) 

For  the  purpose  of  facilitating  the  reading,  special  forms 
of  burettes  are  constructed  which  are  provided  with  a  dark 
longitudinal  stripe  on  a  white  enameled  background  (Fig. 
30);  the  reflection  of  the  dark  stripe  with  the  meniscus  pro- 
duces the  peculiar  appearance  shown  in  Fig.  3 1 .  The  narrowest 
point  is  at  the  middle  of  the  meniscus,  and  by  reading  from 
this  point  very   accurate   measurements   are   obtained.     The 


46       THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

same  effect  can  be  produced  by  holding  behind  an  ordinary- 
burette  a  white  flexible  card  having  a  heavy  black  longitudinal 
stripe,  about  one-eighth  inch  in  width. 

Another  form  of  burette  designed  for  the  purpose  of 
facilitating  reading  is  that  provided  with  white  enameled 
sides,  leaving  a  strip  of  clear  glass  in  front  and  back  (Fig. 
32).  This  form  is  especially  adapted  for  use  with  dark- 
colored  Hquids,  such  as  iodin  and  permanganate. 

CAIIBRATION  OF  INSTRUMENTS 

Burettes  are  made  from  tubes  of  nearly  uniform  width. 
They  are  filled  with  distilled  water  at  15°  C*  (59°  F.)  to  the 
o  mark,  and  then  25,  50  or  100  cc.  run  out,  and  another  mark 
made  at  the  surface  of  the  liquid.  The  distance  between 
these  two  marks  is  then  divided  into  25,  50  or  100  equal 
parts,  and  the  spaces  again  subdivided  into  fifths  and  tenths. 
Now  it  is  very  rarely  possible  to  obtain  tubes  of  exactly  the 
same  caliber  throughout,  and  the  divisions  made  as  above 
do  not  always  represent  exactly  what  they  are  intended  to. 

If  the  tube  is  wider  at  one  point  the  divisions  at  that 
point  will  contain  more,  and  if  it  is  narrower  they  will  contain 
less  than  they  should. 

Hence  before  using  a  new  burette,  or  in  fact  any  other 
measuring  instrument,  it  is  essential  that  the  error,  if  any, 
should  be  determined.     This  is  done  as  follows: 

Fill  the  burette  to  the  o  mark  with  distilled  water  at  15° 
C.  (S9°  F.)  and  run  out  10  cc.  at  a  time  into  a  small  weighed 
flask,  and  weigh  after  each  addition  of  10  cc. 

Each    10   cc.   should   weigh   exactly    10   gms.,   and   every 

*  Instead  of  15°  C.  (59°  F.)  the  temperature  25°  C.  (77°  F.)  is  recommended 
because  this  more  nearly  approaches  the  ordinary  temperature  of  the  atmosphere 
in  temperate  climes. 


ON  THE  USE  OF  APPARATUS 


47 


deviation  found  should  be  noted  and  taken  into  consideration 
in  using  the  instrument. 

Example 
Flask  weighed  20.0000  grams. 


"       +IOCC. 

30.1005    '' 

"     +  20  cc. 

40.0499    " 

"     +30  cc. 

49.8000     " 

''     +40  cc. 

59.9700    " 

"    +50CC. 

70.0100     " 

Thus  the  ist  10.  cc  weighed  10.1005  grams. 

2d  10  cc.       "  9.9494      " 

3d  10  cc.        "  9-7501       " 

4th  10  cc.       ''  10.1700      '' 

5th  10  cc.       ''  10.0400      " 

Having  obtained  these  data,  a  table  like  the  following  may 
be  constructed  and  kept  in  some  convenient  place  where  it 
can  be  readily  consulted  whenever  the  burette  it  represents 
is  being  used.  It  is  not  necessary  to  carry  the  figure  beyond 
the  second  decimal  place. 


No.  of  cc. 

No.  of  cc. 

No.  of  cc. 

No.  of  cc. 

No.  of  cc. 

No.  of  cc. 

as  read  on 

as 

as  read  on 

as 

as  read  on 

as 

burette. 

corrected. 

burette. 

corrected. 

burette. 

corrected. 

I 

1 .01 

14 

14.06 

27 

26.79 

2 

2.02 

15 

15   05 

28 

27.76 

3 

Z-03> 

16 

16.04 

29 

28.73 

4 

4.04 

17 

17-03 

30 

29.70 

5 

5-05 

18 

18.02 

31 

30.71 

6 

6.06 

19 

19.01 

32 

3172 

7 

7.07 

20 

20.00 

2,Z 

32.73 

8 

8.08 

21 

20.97 

34 

33-74 

9 

9.09 

22 

21.94 

35 

34-75 

10 

10.10 

23 

22.91 

36 

35  76 

II 

II  .09 

24 

23.88 

37 

36.77 

12 

12.08 

25 

24.85 

38 

37-78 

13 

13-07 

26 

25.82 

39 

38.79 

48        THE  ESSENTIALS  OF   VOLUMETRIC  ANALYSIS 

There  should  be  no  greater  deviation  than  0.15  cc.  A 
burette  which  deviates  more  is  best  not  used.  In  the 
foregoing  table  there  is  a  deviation  of  0.30  cc.  at  one  point. 

In  order  to  test  the  accuracy  of  a  pipette,  fill  to  the  mark 
with  distilled  water  at  15°  C.  (59°  F.);  empty  into  a  previously 
weighed  flask,  weigh  again  and  thus  determine  the  weight  of 
the  water  measured.     One  gram  is  equal  to  i  cc. 

Liter  flasks  are  tested  as  follows: 

The  flask,  perfectly  dry  and  clean,  is  counterpoised  on  a 
balance  capable  of  turning  with  .005  when  carrying  about 
2000  grams;  it  is  then  filled  to  the  mark  with  distilled  water 
at  15°  C.  (59°  F.)  and  the  increase  in  weight  should  be  exactly 
the  number  of  grams  as  the  cc.  indicated  at  the  mark. 

When  very  accurate  determinations  are  required,  various 
factors  should  be  taken  into  account,  namely,  atmospheric 
pressure,  the  temperature  of  the  air,  and  that  of  the  water, 
which  should  be  the  same.  Atmospheric  humidity  should  be 
of  a  definite  degree,  and  the  weights  used  should  be  made  of 
specified  material. 

The  calibration  of  a  burette,  which  is  graduated  in  mils, 
at  25°  C,  is  conducted  as  follows: 

Fill  the  burette  to  the  o  mark  with  distilled  water  at  25° 
C.  The  air  should  be  half  saturated  with  moisture,  and  of 
the  same  temperature  as  the  water.  The  barometric  pressure 
should  be  760  mm.  The  weights  used  should  be  of  brass,  and 
the  counterpoise  of  the  flask  should  be  of  the  same  kind  of  glass 
as  that  of  which  the  flask  is  made.  Run  out  10  mils  at  a  time 
into  the  small  counterpoised  flask,  and  weigh  each  10  mils. 

Each  10  mils  should  weigh  exactly  9.9604  gms.,  and  every 
deviation  found  should  be  noted  and  taken  into  consideration 
in  using  the  burette. 


ON  THE   USE  OF  APPARATUS 


49 


Example 

ist  lo  mils  weighed    9.9610  gms.  (deviation  negligible) 
2d   10    "  "         9.8972     '' 

3d   10    ^'  ''         9.9543     '' 

4th  10    ''  "        10.0864     " 

5th  10    ''  ''         9.9753     '' 

From  these  data  a  table  like  the  following  may  be  con- 
structed, and  consulted  whenever  the  burette  it  represents  is 
being  used. 


No.  of 

No.  of 

No.  of 

1 
No.  of 

No.  of 

No.  of 

Mils  as 

Mils  as 

Mils  as 

Mils  as 

Mils  as 

Mils  as 

Read  on 

Corrected. 

Read  on 

Corrected. 

Read  on 

Corrected. 

Burette. 

Burette. 

Burette. 

I 



I 

18 

17944 

35 

34-983 

2 

2 

19 

18.937 

36 

35-995 

3 

3 

20 

19.930 

31 

37-008 

4 

4 

21 

20.929 

38 

38.020 

5 

S 

22 

21.928 

39 

39-033 

6 

6 

23 

22.927 

40 

40 . 046 

7 

7 

24 

23.926 

41 

41 . 046 

8 

8 

25 

24.925 

42 

42.046 

9 

9 

26 

25.924 

43 

43 ■ 046 

16 

10 

27 

26.923 

44 

44-046 

II 

10.993 

28 

27.922 

45 

45.046 

12 

11.986 

29 

28.921 

46 

46 . 046 

13 

12.979 

30 

29.920 

47 

47-047 

14 

13.972 

31    . 

30.932 

48 

48.047 

IS 

14  965 

32 

31-945 

49 

49.047 

16 

15-958 

33 

32-957 

50 

50.047 

17 

16.951 

34 

33  970 

CHAPTER  VII 

METHODS  OF  CALCULATING  RESULTS 

N 
Each  mil  of  a  —   univalent  volumetric  solution  contains 

tAtf  of  the  molecular  weight  in  grams  of  its  reagent,  and 

will  neutralize  toVtt  of  the  molecular  weight  of  a  univalent 

substance,   or   ^oVir  of   the   molecular   weight   of   a   bivalent 

substance. 

N 
Each   mil  of  a  —   bivalent  volumetric   solution   contains 
I 

Ywuu  of  the  molecular  weight  in  grams  of  its  reagent,  and  will 

neutralize  or  combine  with  ^^Vt  of  the  molecular  weight  of 

a  bivalent  salt,  or  y^rW  of  the  molecular  weight  of  a  univalent 

salt. 

N  .  • 
A  —  is  only  iV  the  strength  of  a  normal  solution  and  will 

neutralize  only  ^  the  quantity  of  salt,  etc. 

Normal  and  decinormal  solutions  of  acids  should  neutralize 
normal  and  decinormal  solutions  of  alkalies,  volume  for  vol- 
ume. Decinormal  solution  of  silver  nitrate  and  decinormal 
solution  of  hydrochloric  acid  or  sodium  chlorid  should  combine, 
volume  for  volume,  etc. 

Rules  for  Direct  Percentage  Estimations;  i.  With  normal 
solutions  xV  or  ^^  (according  to  its  atomicity)  of  the  molec- 
ular weight  in  grams  of  the  substance  is  weighed  for  titration, 
and  the  number  of  mils  of  the  V.S.  required  to  produce  the 
desired  reaction  is  the  percentage  of  the  substance  whose 
molecular  weight  has  been  used. 

50 


METHODS  OF  CALCULATING  RESULTS  51 

Thus,  if  sodium  hydroxid  (NaOH)  is  to  be  examined  by 
titration  with  a  normal  acid  solution  j\  of  its  molecular 
weight  in  grams,  4  gms.  is  weighed  out,  and  each  mil  of  the 
acid  solution  required  represents  one  per  cent  of  the  pure  salt. 

If  sodium  carbonate  (Na2C03)  is  to  be  titrated  jV  of  its 
molecular  weight  in  grams,  5.3  gms.  is  taken. 

2.  With  decinormal  solutions  li^  or  -1^  of  the  molecular 
weight  in  grams  of  the  substance  to  be  analyzed  is  taken, 
and  the  number  of  mils  will,  in  like  rrianner,  give  the  percentage. 

The  following  equations  will  serve  to  explain  more  fully: 

N 
Sodium  hydroxid  with  —  sulphuric  acid: 

2NaOH  +  H2SO4  =  Na2S04  +  2H2O. 

2X40=80       2)98 

10)40  ^g  ==  1000  mils 

4.0  gms.       =     100  mils 

Sodium  carbonate  with  —  sulphuric  acid: 

Na2C03  +  H2SO4  =  Na2S04  +  H2O  +  CO2. 

2)98 

20)  io6_  ^g  =.  1000  mils 

5.3  gms."  =     100 mils 

N 
With  —  sulphuric  acid: 
10       ^ 

2NaOH  +  H2SO4  =  Na2S04  +  2H2O. 

2X40=80       2)98 

100)40  ^  =  1000  mils 

0.40  gm.     =     100  mils 

In  the  case  of  a  trivalent  substance  as  citric  acid  j^o  oi 
the  molecular  weight  in  grams  is  taken  for  analysis  when  a 
normal  solution  is  employed  and  ^^^  when  a  decinormal 
solution  is  used. 

In  other  words,  when  it  is  desired  that  each  mil  of  the 


52         THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

standard  solution  should  represent  one  per  cent  of  the  substance 

upon  which  it  acts,  the  rule  is  to  take  for  analysis  as  much 

of  the  substance  as  is  represented  by  loo  mils  of  the  standard 

solution. 

In  the  case  of  substances  whose  percentage  of  purity  is 

high,  it  is  advisable  to  take  smaller  quantities,  in  order  to 

avoid   the  use   of  excessive   quantities   of  standard   solution. 

Thus  sulphuric  acid,  which  contains  92.5  per  cent  of  absolute 

sulphuric  acid,  would  require  under  the  above  conditions  92.5 

mils  of  normal  alkali  solution. 

In  the  case  of  this  acid,  if  4.9  gms.  are  taken  for  analysis, 

each  mil  of  a  normal  alkali  solution  would  represent  one  per 

cent  of  H2SO4. 

If   half   of   this   quantity,    i.e.,    2.45    gms.   are   taken   for 

analysis,  each  mil  of  the  normal  alkali  will  represent  two  per 

cent  of  H2SO4,  and  thus  less  of  the  standard  solution  will 

be  required.     Again,  if  0.49  gm.  be  taken,  each  mil  of  the 

standard  alkali  will  represent  10  per  cent  of  H2SO4. 

In  the  case  of  liquids  where  it  is  not  always  convenient 

to  weigh  off  the  exact  quantity  required  for  titration  by  the 

direct  percentage  method,  the  liquid  is  diluted  to  a  convenient 

degree  with  water,  and  then  a  quantity  of  this  diluted  liquid 

(representing  the  weight  required  of  the  substance)  is  measured 

for  analysis. 

Example.    A   sulphuric   acid   solution   of   specific   gravity 

1.826  is  to  be  analyzed.     Two  mils  are  accurately  measured 

and  diluted  to  100  mils  and  then  13.41  mils  of  this  solution 

(representing  0.49  gm.  of  the  acid)  are  taken  for  analysis. 

N 
Each  mil  of  —  NaOH  V.S.  required  in  the  titration  rep- 

N 
resents  10  per  cent  of  absolute  H2SO4.     If  —  NaOH  V.S. 

is  employed,  each  mil  will  represent  one  per  cent.    To  de- 


METHODS  OF  CALCULATING  RESULTS  53 

termine  the  amount  of  the  diluted  liquid  to  be  taken  we 
proceed  as  follows : 

Two  mils  of  sulphuric  acid,  specific  gravity  1.826,  weigh 
3.652  gms.,  therefore  the  100  mils  of  diluted  acid  contain  this 
weight,  and  i  mil  of  the  same  contains  0.03652  gm. 

If  0.03652  gm.  are  contained  in  i  mil,  then  0.49  gm.  are 
contained  in  how  many  mils  ? 

gm.      mil    gm. 

.03652  :  i::o.49  •  ^> 

:v=  13.41  mils. 

Factors  or  Coefficients  for  Calculating  the  Analyses.     It 

frequently  occurs  that  from  the  nature  of  the  substance,  or 
from  its  being  in  solution,  this  percentage  method  cannot  be 
conveniently  followed. 

The  best  way  to  proceed  in  such  a  case  is  to  find  the 
factor. 

The  first  step  in  all  cases  is  to  write  the  equation  for  the 
reaction  which  takes  place  between  the  substance  under 
analysis  and  the  solution  used. 

For  instance,  a  solution  of  caustic  potash  is  to  be  examined, 

N 
a  —  solution  of  sulphuric  acid  being  used. 

2KOH  +  H2SO4  =  K2SO4  +  2H2O. 

2)_II2_  2)98  .       N        . 

56  49    =  1000  mils  —  acid. 

0.56  gm.      .049    =        I  mil  —  add. 

N 
The  factor  for  KOH  when  —  solution  is  used  is  .056  gm., 

N 
that  being  the  quantity  neutralized  by  each  mil  of  the  —  acid. 

N 
If  —  acid  were  used  the  factor  would  be  .0056  gm. 


54        THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

The  number  of  mils  of  the  acid  used  to  produce  the  desired 
result,  when  multiplied  by  the  factor,  gives  the  quantity  in 
grams  of  KOH  in  the  solution  taken. 

Example.     If    lo   grams   of   caustic   potash   solution   were 

N 
taken,  and  40  mils  of  —  acid  were  required,  the   10  gms.  of 

solution  contained  .056  gm.X4o  =  2.24  gms.  of  pure  KOH. 

To  find  the  percentage  the  following  formula  may  be 
used: 

e_xioo_ 

W    ~/^* 

Q  =the  quantity  of  pure  substance  found  by  calculation; 
I^  =  weight  of  substance  taken. 

If  the  above  example  is  taken,  we  have 


2.24X100  ^ 


Or  the  calculation  may  be  made  by  proportion. 

The  quantity  of  the  substance  taken  is  always  the  first 
term,  and  the  quantity  of  pure  substance  found,  the  second 
term. 

The  following  rule  is  easily  remembered:  As  the  quantity 
taken  is  to  the  quantity  found,  so  is  100  to  x,  the  percentage 
of  pure  substance  in  the  sample. 

Three  terms  of  the  equation  being  given,  the  fourth  is 
found  by  multiplying  the  means  and  dividing  the  product  by 
the  given  extreme.  By  applying  this  rule  to  the  above  case 
we  have 

10  :  2.24  :  :  100  :rv.         x  =  22.4%. 


METHODS  OF  CALCULATING  RESULTS 


55 


TABLE    SHOWING    THE    NORMAL    FACTORS,     ETC., 
ALKALIES,  ALKALI  EARTHS,  AND  ACIDS. 


OF    THE 


Substance. 


Formula. 


Molecular 

Normal 

Weight. 

Factor.* 

40 

0.040 

106 

0-053 

84 

0.084 

56.1 

0.0561 

138.2 

0.0691 

100. 1 

0.  lOOI 

17.03 

0.01703 

96.08 

0.0^804 

56.1 

p. 02805 

74.1 

0.03705 

100 . 1 

0.050 

63  .01 

0.063 

36  46 

0.03646 

98.08 

0 . 04904 

1 26 .  05 

0.063 

60  .03 

0 . 0603 

Quantity  of 

Substance 

to  be  taken! 

so  that  each 

mil  of  -V.S. 
I, 

will  indicate 
I  per  cent. 


Sodium  hydroxid 

Sodium  carbonate 

Sodium  bicarbonate 

Potassium  hydroxid 

''potassium  carbonate 

Potassium  bicarbonate 

Ammonia  (gas) 

Ammonium  carbonate,  normal 

Lime 

Calcium  hydroxid 

Calcium  carbonate 

Nitric  acid 

Hydrochloric  acid 

Sulphuric  acid 

Oxalic  acid,  crystallized  .  .  .  . 
Acetic  acid 


NaOH. 

NaaCOg 

NaHCOa 

KOH 

K2CO, 

KHCO3 

NH3 

(NHJ^CO^ 

CaO 

Ca(OH)2 

CaCOa 

HNO3 

HCl 
H2SO, 

H2C20^-2H20 
HC  2fl3v)2 


Grams 
4.0 
5-3 
8.4 
5 -61 
6.91 

10.01 

1-703 

4.804 

2.805 

3-705 

5- 

6.3 

3.646 

4.9 

6.3 
6.03 


*  This  is  the  coefficient  by  which  the  number  of  mils  of  normal  solution  used  is 
to  be  multiplied  in  order  to  obtain  the  quantity  of  pure  substance  present  in  the 
material  examined. 

t  This  is  the  quantity  of  substance  to  be  taken  in  direct  percentage  estimations. 

N 
Each   mil  of  —  acid  or  alkali  V.S.  employed  will  then  indicate  i  per  cent;    in  the  case 

of  many  of  these  substances  it  will,  however,  be  better  to  take  smaller  quantities  so 

that  less  of  the   standard   solution   be  required.     Thus  if  one-half  the  quantity   be 

N 
taken  each  mil  of  the  —  V.S.  will  represent  2  per  cent,  if  one-tenth  of  the  quantity  be 

N 
taken  each  mil  will  represent  10  per  cent.     If.  however,  —  solutions  be  used  and  one- 
tenth  of  the  quantity  indicated  in  the  table  be  taken  each  mil  will  indicate  i  per  cent. 

On  Stating  Results.  In  reporting  the  results  of  volu- 
metric work,  it  is  customary  to  state  the  quantity  of  pure 
substance  found;  thus  in  the  case  of  salts,  the  quantity  of 
the  anhydrous  salt  is  reported.     It  is,  however,  often  required 


56  THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

to  state  the  results  according  to  the  dualistic  formulae  of 
Berzelius,  that  is,  the  metals  are  reported  as  oxids  and  the 
acids  as  anhydrids.  Thus  if  sodium  carbonate  is  analyzed, 
a  statement  of  results  by  this  method  will  give  the  quantity 
of  Na20,  spoken  of  as  the  base,  soda.  If  we  look  upon 
sodium  carbonate  as  Na20C02,  we  can  readily  see  io6  gms. 
of  the  anhydrous  salt  contain  62  gms.  of  Na20. 

By  reference  to  the  following  equations  we  see  that  98.08 
gms.  of  sulphuric  acid  will  neutralize  62  gms.  of  Na20  or 
106  gms.  of  Na20C02. 

NasO     +     H2SO4     =     Na2S04     +     H2O. 

2)62  2)98.08  ^    ^ 

31  gms.  49.04  gms.   =  1000  mils  —  V.S. 


Na20C02  +  H2S04  =  Na2S04+H20  +  C02. 

2)2o6_  ^     j^ 

53  gms.  =  to  1000  mils  —  V.S. 

N 
Thus  one  mil  of  —  H2SO4  will  represent  0.031   gm.    of 

Na20  and  0.053  gm.  of  Na20C02. 

In  the  case  of  sodium  bicarbonate  (NaHCOs)  two  molecules 
contain  one  molecule  of  the  base,  soda,  as  here  shown. 

2NaHC03  =  Na20,H20(C02)2. 

According  to  this  62  gms.  of  Na20  represent  two  molecular 
weights  (168  gms.)  of  NaHCOs.  In  the  case  of  ferrous  sul- 
phate, one  molecule  (FeS04)  contains,  according  to  this  system, 
FeO  and  SO3.  In  the  same  way,  ferric  salts  contain  Fe203. 
In  stating  the  results  of  analyses  of  acids  according  to  this 
system,  the  quantity  of  acid  anhydrid  found  is  reported,  not 
the  quantity  of  the  whole  acid.  Thus  if  sulphuric  acid  is 
analyzed,  the  quantity  of  SO3  is  reported.  In  the  case  of 
phosphoric  acid  the  quantity  of  P2O5  is  stated,  etc. 


METHODS  OF  CALCULATING  RESULTS 


57 


TABLE  SHOWING  THE  MOLECULAR  WEIGHTS  AND  NORMAL' 
FACTORS  FOR  THE  MOST  COMMON  OXIDS. 


Name. 


Formula. 


Molecular 
Weight.* 


N 

—  Factor. 


Soda 

Potash 

Lime 

Magnesia 

Lithium  oxid 

Strontium  oxid  .... 

Barium  oxid 

Zfcic  oxid 

Lead  oxid 

Arsenous  oxid 

Antimonous  oxid  . . . 
Mercurous  oxid  .... 

Mercuric  oxid 

Ferrous  oxid 

Ferric  oxid 

Silver  oxid 

Sulphuric  anhydrid  . 
Phosphoric  anhydrid 
Nitric  anhydrid  .... 
Carbonic  anhydrid  .  , 


Na^O 

K2O 

CaO 

MgO 

U2O 

SrO 

BaO 

ZnO 

PbO 

AS2O3 

Sb^Oj 

HgzO 

HgO 

FeO 

Fe^Og 

AgzO 

SO3 

P2O5 

N2O5 

co; 


62.0 

94.2 

56.1 

40.32 

29.88 

103.63 

153-37 

81.37 
223.1 

197.92 
288.4 
416.0 
216.0 

71.82 
159.64 
231.76 

80.07 
142 .08 
108.02 

44-0 


0.031 
0,0471 
0.028 
0.020 
0.0149 
0.0518 
0.0767 
o .0407 
1115 

0495 

0721 

208 

108 

0718 

0798 

"59 
0.040 
0.02368 
0.054 
0.022 


gm. 


*  Approximate. 


■'■/ 


CHAPTER  Vni 
ANALYSIS  BY  NEUTRALIZATION 

This  is  based  upon  the  fact  that  when  an  acid  and  an 
alkali  react  each  loses  its  individuality  and  a  neutral  salt  is 
formed,  i.e.,  a  body  which  has  neither  the  character  of  an 
acid  nor  that  of  an  alkali.     This  mutual  neutralization  of 

4- 

acid  and  alkali  is  the  result  of  a  union  of  the  H*  ions  of  the 

acid  and  the  OH*  ions  of  the  alkali,  forming  non-ionized  water 
(HOH).  • 

An  acid  is  a  compound  which  in  aqueous  solution  disso- 
ciates  (ionizes)   into  positive  and  negative  ions,  the  positive 

ion  being  H.     Thus  hydrochloric  acid  in  an  ionized  state  is 

+       -  +       -I-        - 

H'  H-Cl*.     Sulphuric  acid  ionizes  into  H*  +H'  +  (SO4). 

An  alkali  is  a  basic  compound  which  ionizes  into  positive 
and  negative  ions,  and  in  which  the  negative  ion  is  (OH). 

The  reaction  between  hydrochloric  acid  and  potassium 
hydroxid.  in  accordance  with  this  theory,  is  illustrated  by 
the  following  equation: 

H  +  Cl  +  K  +  OH  =  K-hCl+HOH. 

An  acid  is  generally  recognized  as  such  by  its  color  reac- 
tions with  certain  substances  known  as  indicators;  for  example, 
it  turns  blue  litmus  red,  and  decolorizes  a  red  phenolphthalein 
solution.      Alkalies  are  recognized  by  their  turning  red  litmus 

68 


ANALYSIS  BY  NEUTRALIZATION  59 

blue,  and  by  producing  a  deep  red  color  with  phenol- 
phthalein. 

The  strength  of  an  acid  solution  is  ascertained  by  noting 
the  quantity  of  alkali  that  is  required  to  neutralize  it.  The 
stronger  the  acid,  the  more  alkali  is  required.  The  strength 
of  an  alkali  is  estimated  by  .  the  quantity  of  acid  which  is 
required  to  neutralize  it.  The  estimation  of  the  strength  of 
acids  is  called  acidimetry,  while  the  estimation  of  alkalies 
is  called  alkalimetry. 

^'he  principal  alkaline  substances  which  may  be  estimated 
by  means  of  standard  acid  solutions  are  the  hydroxids  and 
carbonates  of  sodium,  potassium,  lithium  and  ammonium,  and 
the  hydroxids  and  oxids  of  calcium,  barium  and  strontium 
and  the  alkaloids. 

When  an  acid  is  brought  in  contact  with  an  alkali,  a 
reaction  takes  place  in  which  a  neutral  salt  is  formed.  This 
is  known  as  neutralization,  and  takes  place  between  definite 
and  invariable  proportions  of  the  reacting  bodies;  thus,  if 
1 12.2  parts  of  potassium  hydroxid  are  mixed  with  98.08  parts 
of  absolute  sulphuric  acid,  the  alkah  as  well  as  the  acid  will 
be  exactly  neutralized.  If  only  80  parts  of  the  acid  have  been 
added  the  mixture  would  still  be  alkaline,  for  it  requires  98.08 
parts  of  the  acid  to  neutralize  112.2  parts  of  potassium 
hydroxid.  If  more  than  98.08  parts  of  the  acid  have  been 
added,  the  mixture  would  be  acid,  and  would  consist  of 
potassium  sulphate  and  free  sulphuric  acid.  The  reaction 
is  thus  illustrated: 

2KOH     +     H2SO4     =     K2SO4     +     2H2O. 

2K=78.2  2H=   2.016 

20  =  32.0  8  =  32.070 

2H=   2.016  40  =  64.00 

112. 216  98.086 

Sodium  hydroxid  will  unite  with  oxalic  acid  in  the  propor- 


60         THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

tion  of  80.016  parts  by  weight  of  the  former  and   126.048 
parts  by  weight  of  the  latter,  as  the  equation  shows. 

2NaOH + H2C2O4  •  2H2O  =  Na2C204 +4H2O. 

2Na=46  6H-  =  6.048 

20=32  20  =  24.000 

2H'=   2.016  60  =  96.000 

80.016  126.048 

Ammonia  water  unites  with  hydrochloric  acid  as  per  the 
equation, 

NH4OH +HC1  =  NH4CI +H2O. 

35.05       36.46 

Sodium  carbonate  with  hydrochloric  acid, 

Na2C03  +  2HCI  =  2NaCl + H2O  +  CO2. 
106  72.92 

Upon  a  careful  perusal  of  the  foregoing  equations  it  will 
be  seen  that  since  definite  weights  of  acids  neutralize  definite 
weights  of  alkalies,  the  quantity  of  a  certain  alkali  in  solution 
can  be  easily  determined  by  the  quantity  of  an  acid  solution 
of  known  strength  required  to  neutralize  it,  and  vice  versa. 

Referring  to  the  first  equation  we  see  that  98.086  gms. 
of  H2SO4  neutralize  1 12.216  gms.  of  KOH.  If  we  prepare 
a  normal  solution  of  H2SO4  we  take  half  the  molecular  weight, 
98.086  =  49.043  gms.,  to  1000  mils.  Half  the  molecular  weight 
is  taken  because  sulphuric  acid  is  a  bivalent  acid.  1000  mils 
of  this  solution  will  neutralize  56.108  gms.  of  KOH;  hence 
I  mil  will  neutralize  0.056108  gm.  of  KOH. 

Thus  if  10  gms.  of  a  solution  of  KOH  be  treated  with 
the  above  normal  solution  of  H2SO4,  and  it  is  found  that 
25  mils  of  the  acid  solution  are  required  to  neutralize  the 
alkali  solution,  the  latter  contains  25X0.0561  =  1.40+  gm.  of 
pure  KOH. 

Since  the  acid  and  alkali  as  well  as  the  neutral  salt  which 


ANALYSIS  BY  NEUTRALIZATION  61 

is  formed  are  colorless,  and  no  visible  change  takes  place 
during  the  reaction,  it  is  necessary  to  add  some  substance 
which  by  change  of  color  will  show  when  the  neutralization 
is  complete.     Such  a  substance  is  known  as  an  indicator. 

In  the  case  of  sodium  hydroxid  with  oxalic  acid  (see  the 
second  equation)  we  find  that  126.048  gms.  of  crystallized 
oxalic  acid  neutralizes  80.016  gms.  of  NaOH.  Oxalic  acid, 
like  sulphuric,  is  bivalent,  therefore  a  normal  solution  of  it 
conta&s  half  the  molecular  weight  in  grams,,  i.e.,  63.024  gms. 
in  1000  mils. 

1000  mils  will  neutralize  40  gms.  of  NaOH; 
I  mil  will  neutralize  0.040  gm.  of  NaOH. 

The  neutralizing  power  of  all  normal  acids  is  exactly  the 
same,  because  they  all  contain  in  1000  mils  the  molecular 
weight  in  grams  of  the  acid  in  the  case  of  univalent  acids, 
and  half  of  the  molecular  wieght  in  grams  of  bivalent 
acids. 

Thus  I  mil  of  any  normal  acid  will  neutralize  0.0561  gm. 
of  KOH  or  0.040  gm.  of  NaOH  or  ywtt  oi  the  molecular 
weight  of  any  other  univalent  alkali,  or  ^^g^  of  the  molecular 
weight  of  an  alkali  earth,  the  latter  being  bivalent.  In  like 
manner  all  decinormal  solutions  have  a  like  neutralizing 
power,  their  neutralizing  equivalence  is  one-tenth  that  of 
normal  solutions. 

Thus  I  mil  of  a  decinormal  acid  will  neutralize  0.00561 
gm.  of  KOH  or  0.0040  gm.  of  NaOH,  etc. 

Alkalimetry 

Preparation  of  Standard  Acid  Solutions.  It  is  possible 
to  carry  out  the  titration  of  most  alkalies  by  means  of  one 
standard  acid  solution,  but  the  same  standard  acid  is  not 
equally  applicable  in  all  cases;    furthermore,   the  standard 


62        THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

acids  are  frequently  employed  for  other  volumetric  operations 
than  neutralization,  and  therefore  it  is  advisable  to  have  a 
variety. 

The  standard  oxalic  acid  solution  is  preferred  by  some, 
because  of  the  ease  with  which  it  may  be  prepared,  provided 
a  pure  oxalic  acid  is  to  hand.  It  does  not,  however,  keep 
very  long,  is  unrehable  for  use  with  methyl  orange,  and  is 
inapplicable  for  the  titration  of  alkali  earths,  because  it  forms 
insoluble  compounds  with  these  metals.  Standard  hydrochloric 
acid  is  the  most  desirable  for  alkali  earths,  because  it  forms 
soluble  compounds  with  them;  its  disadvantage,  however,  is 
in  its  volatility  and  its  consequent  uselessness  in  hot  titrations. 
Standard  sulphuric  acid  is  preferred  by  most  analysts  as 
being  the  best  general  standard.  A  pure  acid  can  be  gotten 
without  difficulty  and  the  standard  solution  made  from  it  is 
unaffected  by  boiling,  and  can  therefore  be  used  in  hot  as 
well  as  in  cold  titrations;  it  reacts  sharply  with  the  indicators 
and  it  keeps  its  titer  indefinitely.  It  is,  however,  not  suited 
for  the  titration  of  alkali  earths,  because  it  forms  insoluble 
compounds  with  them,  which  precipitate  and  are  very  annoying 
to  the  operator.  I;i  the  preparation  of  standard  solutions  the 
greatest  care  should  be  exercised  in  order  that  the  product 
be  absolutely  accurate.  The  sHghtest  inaccuracy  in  the 
strength  of  a  standard  solution  will  result  in  relative  errors 
in  the  analysis.  It  is  customary  to  prepare  one  standard 
solution,  and  then  from  this  to  adjust  various  others.  For 
example,  a  normal  oxalic  acid  may  be  made  first,  and  by 
means  of  this  a  normal  alkali  solution,  which  in  turn  may 
be  utilized  for  the  adjusting  of  other  standard  acid  solu- 
tions. 

N 
Normal  OxaUc  Acid  V.S.  (H2C204-2H20  =  126.05,  —V.S. 

=  63.025  gms.  in  1000  mils).     Dissolve  63.025  gms.  of  purified 


CALIFORNIA   COLLPftf 

of   PHARMACY 

ANALYSIS  BY  NEUTRALIZATION  63 

oxalic  acid  *  in  enough  distilled  water  to  make,  at  or  near 

25°  C,  exactly  1000  mils. 

/N 
Tenth-Normal    Oxalic  Acid  V.S.    I —  V.S.=  6.3025  gms. 

in  1000  mils.) 

Dissolve  6.3025  gms.  of  purified  oxalic  acid  in  sufficient 
distilled  water  to  make,  at  or  near  25°  C,  exactly  1000  mils 
of  solution.  Or,  better:  dissolve  6.45  gms.  of  pure  crystal- 
lized oifelic  acid  in  sufficient  distilled  water  to  measure  1000 
mils.  Then  into  a  flask  accurately  measure  25  mils  of  a  freshly 
standardized  tenth-normal  potassium  hydroxide  V.S.,  dilute 
with  an  equal  volume  of  distilled  water,  add  3  to  5  drops  of 
phenolphthalein  T.S.  and  heat  it  to  boiling.  From  a  burette 
gradually  add  the  oxalic  acid  solution  (which  is  too  concen- 
trated) until  the  red  tint  of  the  alkali  solution  fails  to  re- 
appear after  vigorous  shaking  and  boiling.  Note  the  num- 
ber of  mils  of  the  oxalic  acid  solution  consumed,  and  then 
dilute  it  so  that  equal  volumes  of  this  and  of  the  tenth- 
normal potassium  hydroxid  V.S.  neutralize  each  other  at 
standard  temperature  (25°  C).  This  solution  deteriorates 
on  standing,  hence  must  be  frequently  renewed  or  restand- 
ardized. 

This  solution  is  in  every  respect  equivalent  in  neutralizing 
power  to  any  other  tenth-normal  acid  V.S.  with  either  litmus 
or  phenolphthalein  T.S.  as  indicator,  but  not  with  methyl 
orange.  Its  specific  use  in  the  U.  S.  P.,  however,  is  in  stand- 
ardizing or  determining  excess  of  tenth-normal  potassium 
permanganate  V.S. 

*  Purified  oxalic  acid  is  in  the  form  of  colorless  transparent,  uneffloresced 
clinorhombic  crystals,  soluble  in  10  parts  of  cold  water,  in  about  3  parts  of 
boiling  water  and  in  2.5  parts  of  alcohol.  Ten  gms,  when  ignited  upon  plati- 
num foil  leaves  not  more  than  0.005  S^-  of  residue.  If  more  residue  is  left 
the  acid  should  be  purified  by  recrystallization. 


64        THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 


Normal   Hydrochloric   Acid   V.S.    (HC1  =  36.47;    7    V.S. 

=  36.47  gms.  in  1000  mils).  Mix  no  mils  of  hydrochloric  acid 
of  sp.gr.  1.155  with  enough  water  to  measure,  at  or  near 
25°  C,  1000  mils. 


EiG.  33. 

Of  this  liquid   (which  is  still  too  concentrated)   measure 

carefully,  at  25°  C,  into  a  flask  or  beaker  10  mils,  add  20 

mils  of  distilled  water  and  a  few  drops  of  methyl  orange  T,S., 

and  then  gradually  add  from  a  burette  sufficient  recently  pre- 

N  '  ,  -  1  I 

pared   and   standardized   —   potassium   or   sodium   hydroxid 


ANALYSIS  BY  NEUTRALIZATION  65 

at  the  same  temperature  to  just  produce  a  permanent  faint 

yellow  tint. 

N 
Note  the  number  of  mils  of  —  alkali  solution  consumed 

I 

and  then  dilute  the  acid  solution  so  that  equal  volumes  of 

N 
it  and  —  alkali  neutralize  each  other.     It  is  usually  advisable 

to  mak^  two   or   three   titrations,   as   just   described,   before 

dilution,  taking  an  average  of  the  results. 

Example.    Assuming  that  the  lo  mils  of  the  acid  solution 

N 
required  12  mils  of  the  —    alakli,  each   10   mils  of   the  acid 

must  be  diluted  to  12  mils,  or  the  whole  of  the  remaining  acid 
in  the  same  proportion. 

After  the  dilution  a  new  trial  should  be  made.  10  mils 
of  the  acid  V.S.  should  required  exactly  10  mils  of  the  alkali. 

This  method  is  faurly  satisfactory  if  an  accurately  stand- 
ardized normal  alkali  hydroxid  solution  is  at  hand;  the  latter, 
however,  always  contains  a  small  qunatity  of  carbonate,  hence 
methyl  orange  is  most  desirable  as  an  indicator. 

Standardization  by  Means  of  Sodium  Carbonate,  Pure 
anhydrous  sodium  carbonate  may  be  obtained  by  heating 
to  dull  redness  a  few  grams  of  pure  sodium  bicarbonate  for 
about  thirty  minutes.  The  resulting  carbonate  is  practically 
free  from  impurity. 

The  sodium  bicarbonate  loses  on  ignition  one-half  of  its 
carbonic  acid  gas : 

2NaHC03+Heat=Na2C03  +  C02+H20. 

The  bicarbonate  should,  however,  be  tested  before  igniting, 
and  if  more  than  traces  of  chlorid,  sulphate,  or  thiosulphate, 
are  found,  these  may  be  removed  by  washing  a  few  hundred 
grams,  first  with  a  saturated  solution  of  sodium  bicarbonate, 
and  afterward  with  distilled  water. 


66        THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

0.53  gm.  of  the  pure  anhydrous  sodium  carbonate  is 
accurately  weighed  and  dissolved  in  about  200  mils  of  dis- 
tilled water  in  a  flask  and  a  few  drops  of  methyl  orange  T.S. 
added  as  indicator.  The  acid  to  be  "  set  "  of  "  standardized  " 
is  then  run  into  the  sodium  carbonate  solution  until  a  per- 
manent light-red  color  is  produced.     It  should  require  exactly 

N 

10  mils  of  the  —  acid  solution. 

I 

If  8  mils  of  the  acid  solution  are  consumed  to  bring  about 

the  required  result,  then  every  8  mils  must  be  diluted  to  10 

mils,  or  the  whole  of  the  remaining  solution  must  be  diluted 

in  this  proportion : 

NasCOa   +   2HCI   =   2NaCl   +  H2O   +  CO2. 

2)106  2)72.9 

S3  gms.  36.45   =   1000  mils  —  V.S. 

0.53  gm.  =       10  mils  I 

Instead  of  attempting  to  weigh  exactly  0.53  gm.  of  the  an- 
hydrous sodium  carbonate,  it  is  better  to  take  a  larger  quantity 
(about  2  gms.)  weighed  accurately.  Dissolve  this  in  100  mils 
of  distilled  water,  add  2  or  3  drops  of  methyl  orange  T.S. 
and  run  into  it,  little  by  little,  from  a  burette  the  acid  solution 
to  be  standardized,  reducing  the  flow  to  drops  toward  the  end 
until  a  pink  color  is  obtained.  Make  2  or  3  trials,  and  take 
the  average  number  of  mils  consumed. 

One  gm.  of  pure  anhydrous  sodium,  carbonate  requires 
for  exact  neutralization  18.868  mils  of  normal  acid  V.S. 

Assuming  that  2.3  gms.  of  the  anhydrous  carbonate  were 
taken,  and  this  required  38.2  mils  of  the  trial  acid.    Then 

2.3  X  18.868  =  43.3964  mils. 

43.3964  :  38.2::  100  :  :r.    ^=88.26. 

Now  adjust  the  acid  by  measuring  882.6  mils  and  dilut- 
ing with  distilled  water  to  make  1000  mils. 


ANALYSIS  BY  NEUTRALIZATION  67 

This  method  may  be  employed  as  well  for  the  standardization 
of  sulphuric  or  oxalic  acid. 

Other  Methods  for  standardizing  hydrochloric  acid  V.S. 

are:    (a)  by  means  of  silver  nitrate  (gravimetrically  and  volu- 

metrically);     (b)   by  means  of  borax;     (c)   by  means  of  the 

specific  gravity;  (d)  by  means  of  calc-spar. 

*  '    N 

Normal    Sulphuric  Acid  V.S.    (H2S04  =  98.09;    -  V.S.= 

49.045' gms.  in  1000  mils).  Mix  carefully  30  mils  of  pure 
concentrated  sulphuric  acid  (sp.gr.  1.835)  with  enough  water  to 
make  about  1050  mils,  and  allow  the  liquid  to  cool  to  about 
25°  C. 

Titrate  10  mils  of  this  liquid  in  the  manner  described  under 

N 

—  hydrochloric  acid,  and  dilute  it  so  that  equal  volumes  of 

the  acid  and  the  alkali  will  neutralize  each  other. 

The  standardization  of  the  normal  sulphuric  acid  solution 
may  also  be  effected  by  the  use  of  pure  anhydrous  sodium 
carbonate,  as  described  under  normal  hydrochloric  acid  V.S., 
and  by  various  other  methods,  among  which  are:  (a)  the 
iodometric;  (b)  the  specific  gravity  method;  (c)  the  borax 
method;  (d)  by  precipitation  with  barium  chlorid  (gravi- 
metrically). 

Standard  acid  solutions  are  used  in  other  strengths  besides 

N  N 

normal,  namely.  Half -normal  — ,  Fifth  normal  — ,     Tenth-nor- 

N  N  N 

m^l  — ,  Twentieth-nor}nal  —  ,  Fiftieth-normal  — ,  and  Hundredth- 
10'  20'     ^  50' 

normal  — . 
100 

Estimation  of  Alkali  Hydroxids 

Potassium  and  sodium  hydroxids  are  usually  titrated  with 

N 

—  sulphuric  or  hydrochloriciacid;   they  are,  however,  so  prone 

to  absorb  carbon  dioxid  out  of  the  air  that  they  are  seldom 
free  from  carbonate,  and  hence  the  selection  of  an  indicator 


68        THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

is  a  matter  of  some  importance.  Phenolphthalein  or  litmus 
may  be  employed,  but  it  is  then  advisable  to  boil  the  solu- 
tion while  titrating,  in  order  to  drive  off  the  liberated  carbon 
dioxid,  because  the  latter  gives  an  acid  reaction  with  phenol- 
phthalein and  litmus  and  thus  causes  an  end-reaction  tint  to 
appear  before  neutralization  is  complete.  It  is  better,  usually, 
to  employ  an  indicator  which  is  not  affected  by  carbon  dioxid. 
Methyl  orange  is  mostly  preferred;  cochineal  and  Congo  red 
are  also  useful.  These  indicators  are  especially  serviceable 
in  the  presence  of  carbonates  in  that  they  are  not  affected  by 
carbon  dioxid,  and  can  therefore  be  employed  in  direct  titra- 
tions without  the  use  of  heat. 

The  quantity  of  carbonate  in  a  recent  sample  of  sodium 
or  potassium  hydroxid  is  so  small  usually  that  it  is  customary 
to  disregard  it  and  to  report  the  total  alkalinity  as  hydroxid. 

A  definite  quantity  of  the  sample  (from  0.5  gm.  to  i  gm. 
of  the  solid  or  an  equivalent  of  a  solution)  is  taken  for  analysis, 
dissolved  in  30  to  50  mils  of  water  in  a  white  porcelain  dish  or 
a  beaker  placed  over  a  white  surface,  and  a  few  drops  of  a 
suitable  indicator  added. 

The  vessel  is  then  placed  beneath  a  burette  containing 
the  standard  acid  solution  and  the  latter  run  in,  drop  by  drop, 
until  the  last  drop  just  causes  the  color  to  change.  The 
solution  should  be  rotated  or  stirred  after  each  addition  of 
the  standard  acid. 

The  alkah  hydroxids  are  so  exceedingly  hygroscopic  that 
they  take  up  water  from  the  air  while  being  weighed;  it  is 
therefore  difficult  to  make  a  direct  weighing  with  any  degree 
of  accuracy. 

The  best  way  is  to  take-  a  small  piece  of  the  sample  (about 
I  gm.),  place  it  immediately  in  a  tared  stoppered  flask  and 
take  the  weight  accurately.  It  is  then  dissolved  in  water, 
transferred  to  the  porcelain  dish  or  beaker  and  titrated. 


ANALYSIS  BY  NEUTRALIZATION  69 

Potassium  Hydroxid  (KOH  =  56.1).    An  accurately  weighed 

portion  (preferably  less  than  i  gm.),  is  placed  in  a  small  beaker, 

dissolved  in  50  mils  of  water,  three  drops  of  methyl  orange 

N 
added,  and  the  titration  begun  with  — ,  sulphuric  acid  and 

continued  until  the  yellow  color  of  the  solution  is  changed  to 
pale  red.  Then  the  burette  is  carefully  read  to  see  how  much 
of  the  acid  solution  was  used.  The  number  of  mils  of  the 
latter  are  multiplied  by  the  normal  factor  for  KOH  (0.0561) 
and  the  result  is  the  quantity  of  pure  KOH  in  the  sample 
taken  for  analysis. 

The  following  equation  illustrates  the  reaction: 

2KOH  +  H2SO4  =  K2SO4  +  2H2O. 
2)112.2  2)98.09  j^ 

56.1  gms.       49.045  gms.,  quantity  in  1000  mils  of  —  acid  V.S. 
0.0561  gm.  (the  factor  for  KOH),  quantity  neutralized  by 

I  mil  of  —  acid, 

I 

N 
Thus  1000  mils  of  —  H2SO4  V.S.  containing  49.045  gms. 

of  absolute  H2SO4  will  neutralize  56.1  gms.  of  KOH.    There 

N 
fore  each  mil  of  —  H2SO4  V.S.  will  neutralize  0.0561  gm.  ol 

pure  KOH. 

Example.  In  the  above  analysis  let  it  be  assumed  that 
0.915  gm.  of  potassium  hydroxid  were  taken  and  that  15.3 
mils  of  the  standard  acid  were  required  to  neutralize  it,  then 
0.0561  gm.X  15.3  =  0.8583  gm.,  the  quantity  of  pure  KOH  in 
the  0.915  gm.  taken. 

The  percentage  is  then  calculated  in  this  way: 

0.915  :  0.8583 ::  100  :  x\       :x:=93.8+per  cent. 

0.8583X100 

=03.0. 

0.915  ^^ 


70        THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

The  fixed  alakli  hydroxids  are  rarely  free  from  carbonate, 
through  absorption  of  carbon  dioxid  from  the  air. 

In  the  foregoing  method,  the  carbonate  is  calculated  as 
hydroxid,  hence  the  assay  is  fallacious.  The  method  of  the 
U.  S.  P.  IX  separates  the  carbonate  by  precipitating  with  barium 
chlorid,  filtering,  and  titrating  the  filtrate  with  normal  hydro- 
chloric acid  V.S.,  using  phenolphthalein  as  indicator.  The 
use  of  standard  sulphuric  acid  V.S.  is  inadmissible  in  this 
method  because  it  reacts  with  barium  chlorid,  producing  a 
precipitate  of  barium  sulphate. 

The  U.  S.  P.  Method  is  as  follows: 

Take  about  lo .  gms.  of  potassium  hydroxid,  accurately 
weighed  in  a  glass-stoppered  weighing  bottle,  dissolve  in  250 
mils  of  distilled  water,  which  has  been  previously  boiled  and 
cooled  (to  expel  carbon  dioxid),  add  30  mils  of  barium  chlorid 
T.S.  and  then  dilute  with  distilled  water  to  500  mils.  Thor 
oughly  agitate  the  liquid  and  filter  it  through  a  dry  filter  into 
a  dry  flask.  Reject  the  first  20  mils,  and  titrate  200  mils  of 
the  clear  filtrate  with  normal  hydrochloric  acid  V.S.,  using 
phenolphthalein  T.S.  as  indicator. 

Sodium  Hydroxid  (NaOH  =  4o).  This  is  estimated  in 
exactly  the  same  manner  as  described  for  potassium  hydroxid, 
the  following  equation  being  applied : 

2NaOH  +  H2SO4  =  Na2S04  +  2H2O. 

2)80  2)98.09  j^ 

40  gms.  49.045  gms.  =  1000  mils  —  V.S. 

N 
.040  gm.  I  mil  —  V.S. 

The  factor. 

The  official  solutions  of  potassium  and  of  sodium  hydroxid 
are  estimated  in  this  same  manner,  10  mils  may  be  taken  for 
analysis,  diluted  with  20  mils  of  water. 

Ammonia  Water  (NH3-II20).  Three  mils  of  ammonia 
water  are  put  into  a  stoppered  weighing  bottle  and  the  weight 


ANALYSIS  BY  NEUTRALIZATION  71 

taken.     Forty  mils  of  water  are  then  added  and  the  solution 

N 
titrated  with  —  sulphuric  acid.    As  indicator,  litmus,  methyl 

orange  or  rosolic  acid  may  be  used.  Phenolphthalein  is  use- 
less for  titrating  ammonia,  and  even  methyl  orange  and  rosolic 
acid  are  unsuitable  in  the  presence  of  much  salts  of  ammo- 
nium. Because  of  the  volatile  character  of  ammonia  its  solu- 
tions readily  lose  strength  upon  exposure.  It  is  therefore  best 
to  measure  a  quantity  into  a  weighing  bottle  and  find  its 
weight  as  directed  for  potassium  hydroxid.  If  the  specific 
gravity  of  the  ammonia  solution  is  known,  the  weight  of  a 
given  volume  is  easily  calculated,  it  being  only  necessary  to 
multiply  the  volume  in  mils  by  the  sp.gr.  Thus,  if  the  sp.gr. 
of  an  ammonia  solution  is  0.9585  and  the  volume  taken  is 
3  mils,  the  weight  of  the  3  mils  is  3X0.9585  =  2.8755  gms. 

N 
In  the  titration  with  —  sulphuric  acid  each  mil  of  the 

latter  represents  0.017  gm.  of  NH3,  as  shown  by  the  equation 

2NH3.H20-hH2S04=(NH4)2S04  +  2H20. 

2)34  2)98.09  J^ 

17  gms.  49-045  gms.  =  1000  mils  —  V.S. 

N 
.017  =1  mil  —  V.S. 

Factor.  ^ 

N 
If  16.9  mils  of  —  acid  were  required  in  the  above  assay, 

then  0.017  gm.X  16.9  =  0.2873  gm.,  the  quantity  of  pure  NH3 
in  the  3  mils  (2.8755  gms.)  of  ammonia  water  taken. 

The  percentage  is  found  as  follows : 

If  3  mils  of  ammonia  water  weighing  2.8755  gms.  contain 
0.2873  gm.  of  NH3,  100  gms.  of  ammonia  water  will  contain 
Xgm.  of  NH3, 

0.2873X100 


2-8755 


=  9.99  per  cent. 


72        THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

Stronger  ammonia  water  and  spirit  of  ammonia  may  be 
estimated  in  the  same  manner. 

Estimation    of   Alkali   Carbonates 

When  carbonates  are  treated  with  acids  carbonic-acid  gas 
is  Hberated.  This  gas  shows  an  acid  reaction  with  most 
indicators,  and  the  reaction  will  seem  to  be  completed  before 
the  alkali  is  entirely  neutralized. 

To  avoid  this,  the  titration  may  be  conducted  at  the  boiling 
temperature  {hot  way)  in  order  to  drive  off  the  carbon  dioxid. 
The  standard  acid  being  added  until  two  minutes'  boiling  fails 
to  restore  the  color  indicating  alkalinity.  If  the  titration  is 
conducted  at  a  boiling  temperature,  it  is  advisable  to  attach 
to  the  lower  end  of  the  burette  a  long  rubber  tube  with  a 
pinch- cock  fixed  about  midway  on  the  tube. 

The  boiling  can  then  be  done  at  a  little  distance  from 
the  burette  and  the  expansion  of  the  standard  solution  therein 
thus  prevented. 

Another  method  is  to  add  to  the  carbonate  a  measured 
excess  of  the  standard  acid,  and  then  after  boiling  to  drive 
off  the  carbon  dioxid,  an  indicator  is  added,  and  the  excess 
of  standard  acid  determined  by  titration  with  a  standard 
alkali  {residual  titration  way).  The  quantity  of  the  latter, 
deducted  from  the  quantity  of  the  standard  acid  taken,  gives 
the  quantity  of  the  latter  which  reacted  with  the  carbonate. 
Still  another  method  is  to  titrate  the  carbonate  direct,  without 
heat  {cold  way),  using  an  indicator  which  is  not  affected  by 
caibon  dioxid.  The  best  of  the  indicators  which  are  not  so 
affected  is  methyl  orange ;  others  are  cochineal  and  Congo  red. 
When  employing  methyl  orange  as  an  indicator  standard  oxalic 
acid  solution  should  not  be  used,  as  the  end-reaction  is  very 
indefinite  and  unreliable. 

The  end-reaction  with  this  indicator  is  at  all  events  not  a 
clearly  marked  one,  and  considerable  practice  and  an  eye  for 
color  is  required  to  detect  the  point  at  which  yellow  changes  \q 


ANALYSIS  BY  NEUTRALIZATION 


73 


pale  pink.  It  is  a  good  plan  to  have  on  the  bench  two  vials, 
one  containing  an  acid  and  the  other  an  alkali  tinted  with 
methyl  orange,  with  which  comparisons  can  be  made. 


Fig.  34 


Potassium  Carbonate  (K2C03=  138.2).  Weigh  carefully 
one  gram  of  the  salt,  previously  dried  to  constant  weight  at 
180°  C.  Dissolve  in  25  mils  of  distilled  water  in  a  beaker 
or  flask,  add  a  few  drops  of  methyl  orange  T.S.,  and  titrate 
with  normal  sulphuric  acid  until  a  faint  orange-red  color 
appears. 

CO2. 


K2CO3 

2)138.2 


+  H2SO4  =  K2SO4  +  H2O  + 

2)98.09  j^ 


69.1  gms.       49.045  gms.  =  1000  mils  —•  V.S. 


N 


Each  mil  of  —  H2SO4,  therefore,  reoresents  0.0691  gm. 

of  pure  potassium  carbonate. 

If  14.3  mils  of  the  normal  acid  are  required  the  salt  contains 


74        THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

14.3X0.0691  gm.  =0.98813  gm.  of  pure  K2CO3  or  98.813 
per  cent.  If  it  is  desired  to  use  litmus  or  phenolphthalein, 
it  will  be  necessary  to  boil  the  solution  as  described  above. 

Other  alkali  carbonates  are  estimated  in  exactly  the  same 
manner  as  described  for  potassium  carbonate. 

Potassium  Bicarbonate  (KHC03=  loo.i). 

2KHCO3  +  H2SO4  =  K2SO4  +  2H2O  +  CO2. 

2)200.2  2)98.09  -VT 

loo.i  gms.        49.045  gms.  =  iooo  mils  —  V.S. 

N 
Each  mil  of  —  acid  V.S.  =  0.1001  gm.  of  KHCO3. 

Previous  to  weighing,  this  salt  should  be  dried  to  constant 
weight  in  a  desiccator  over  sulphuric  acid. 

Sodium  Carbonate  (crystallized)  (Na2C03-  ioH20  =  286.16). 

Na2C03-ioH20   +  H2SO4  =  Na2S04  +  11H2O   +  CO2. 


2)286,16  2)98.09  ^ 

143.08  gms.  49-045  gms.  =  1000  mils  —  V.S. 

N 
Each  mil  of  —  acid  =  0.143  gm.  crystallized  sodium  car- 
bonate. 

Sodium  Carbonate  (anhydrous)  (Na2C03=  106). 


Na2C03  +  H2SO4  =  Na2S04  +  H2O  +  CO2. 

2)106  2)98.09  j^ 

53  gms.  49  045  gms.  =  1000  mils  —  V.S. 

N 
Each  mil  of  —  acid  =  0.053  g"^-  Na2C03. 

Sodium  Bicarbonate  (NaHC03  =  84). 

2NaHC03   +  H2SO4  =  Na2S04  +  2H2O   +  2CC2. 

2)168  2)98.09  -Kx 

84  gms.  49-045  gms.  =  1000  mils  —  V.S. 

N 
Each  mil  of  —  acid  =  0.084  gi^-  NaHCOs. 


ANALYSIS  BY  NEUTRALIZATION  75 

Previous  to  weighing,  the  salt  should  be  dried  to  constant 
weight  in  a  desiccator  over  sulphuric  acid. 
Lithium  Carbonate  (Li2C03  =  73-88)- 

LisCOs  +  H2SO4  =  Li2S04  +  H2O   +  CO2. 

2)73-88  2)98.09  j^ 

36.94  gms.       49.045  gms.  =  1000  mils  —  V.S. 

N 
Each  mil  of  —  acid  =  0.03694  gm.  Li2C03. 

The  U.S.P.  IX  recommends  residual  titration  for  this  salt. 
Previous  to  weighing  it  should  be  dried  to  constant  weight  at 
100°  C.  1.5  gm.  is  dissolved  in  50  mils  of  normal  sulphuric 
acid  V.S.  and  the  solution  titrated  with  normal  potassium 
hydroxid  V.S.,  using  methyl  orange  T.S.  as  indicator. 

Ammonium  Carbonate  (N3HiiC205=  157.03).  Normal 
ammonium  carbonate  has  the  formula  (NH4)2C03,  but  the 
normal  salt  loses  upon  exposure  NH3  and  H2O.  The  commer- 
cial salt,  therefore,  generally  is  a  mixture  of  bicarbonate  and 
carbamate. 

(NH4)2C03  -  NH3  =  NH4HCO3 ; 

(NH4)2C03  -  H2O  =  NH4NH2CO2. 

The  commercial  carbonate  is  therefore  generally  expressed 
thus: 

NH4HCO3.NH4NH2CO2        or        N3H11C2O5. 

This  salt  may  be  estimated  by  direct  titration  with  normal 
or  decinormal  acid,  using  rosolic  acid  or  methyl  orange  as 
an  indicator. 

Two  grams  of  the  salt  are  taken,  dissolved  in  about  50 

N 
mils  of  water  and  titrated  with  —  H2SO4  V.S.     The  reaction 

is  as  follows : 

2N3HnC205  +  3H2SO4  =  3(NH4)2S04  +  4CO2  +  2H2O. 

6)314-06  6)294.18  j^ 

52.34  gms.  49-045  gms.  =  1000  mils  —  acid  V.S. 


76       THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

=  N 

Each  mil  of  —  acid  V.S.  represents  0.052  gm.  of  N3H11C2O5 

or  0.017  gm.  of  NH3.    The  U.S. P.  salt  should  contain  between 
30  and  32  per  cent  of  NH3. 

If  in  this  titration  37.3  mils  of  the  standard  acid  are  required 
then  the  two  grams  of  ammonium  carbonate  contained  0.052 
gm.X37.3  =  1.939  gms.  of  the  salt. 

1.939  X 100 


2 


=  96.95  per  cent. 


If  rosolic  acid  is  used  as  indicator  heat  must  be  applied 
to  expel  carbon  dioxid.  The  estimation  of  the  carbonic  acid 
may  be  effected  by  precipitating  a  definite  weight  of  the  salt 
with  barium  chlorid,  collecting  the  precipitated  barium  car- 
bonate, dissolving  it  in  a  measured  excess  of  normal  hydro- 
chloric acid  and  retitrating  with  normal  alkali. 

The  method  usually  employed  by  skilled  analysts  {the 
residual  titration  method),  is  to  add  a  measured  excess  of  the 
standard  acid  solution,  and  thus  convert  the  ammonium  car- 
bonate into  the  less  volatile  ammonium  sulphate;  then  gently 
boil  to  get  rid  of  CO2,  and  titrate  back  with  a  standard  alkali 
V.S.  (using  methyl  orange  as  an  indicator)  until  the  excess  of 
acid  is  neutralized.  The  quantity  of  free  acid  thus  found, 
when  deducted'  from  the  amount  of  acid  first  added,  gives  the 
quantity  which  was  required  to  neutralize  the  ammonium 
carbonate. 

Thus  2  gms.    (in  solution)   of  ammonium  carbonate  are 

N 
treated  with  50  mils  of  —  H2SO4  V.S.,    which  is  more  than 

sufficient  to  neutralize  it;    the  solution  is  then  gently  boiled 

to  drive  off  CO2,  a  few  drops  of  litmus  tincture  added,  and 

N 
then  titrated  with  —  KOH  V.S.  until  the  litmus  no  longer 

shows  an  acid  reaction  and  the  solution  is  neutral.     If  methyl 
orange  is  used  as  an  indicator  here,  boiling  is  not  necessary. 


ANALYSIS   BY  NEUTRALIZATION  77 

N 
Let  us  assume  that  12.7  mils  of  the  —  KOH  V.S.  were 

'  I 

*  N 

used.     By  deducting  the  12.7  mils  from  the  50  mils  of  —  acid 

first  added,  we  find  37.3  mils  of  the  acid  went  into  combination 
with  the  ammonium  salt,  the  calculation  is  then  made  as 
described  above. 

Mixed  Alkali  Hydroxid  and  Carbonate 

If  it  is  desired  to  ascertain  the  proportion  in  which  these 
exist  in  a  mixture,  we  proceed  as  follows: 

First  determine  the  total  alkalinity  by  means  of  normal 
hydrochloric  acid,  using  methyl  orange  as  an  indicator.  Then 
dissolve  a  like  quantity  of  the  mixture  in  150  mils  of  water 
and  add  sufficient  barium  chlorid  to  precipitate  all  of  the 
carbonate  as  barium  carbonate,  and  then  add  water  to  make 
200  mils  and  set  aside  to  settle.  When  the  supernatant  liquid 
is  clear  take  one-fourth  (50  mils)  of  it,  and  titrate  with  normal 
hydrochloric  acid,  using  phenolphthalein  as  indicator.*  The 
number  of  mils  multiplied  by  4  will  be  the  quantity  of  normal 
acid  required  by  the  caustic  alkali.  The  difference  between 
this  and  the  number  of  mils  representing  the  total  alkalinity 
is  calculated  as  carbonate. 

Example.  Assuming  that  we  are  analyzing  a  mixture  of 
sodium  hydroxid  and  carbonate. 

Two  grams  of  the  substance  are  dissolved  in  water  and 
titrated  with  normal  acid  solution.  43.2  mils  of  the  latter  are 
required.  Another  2  gms.  is  dissolved,  treated  with  barium 
chlorid  as  directed,  and  one-fourth  of  the  clear  solution  titrated 

*  The  slight  error  which  occurs  in  this  method  because  the  volume  of  the 
precipitate  is  included  in  the  measured  liquid,  may  be  overcome  by  using 
the  entire  quantity  of  liquid,  including  the  precipitate  (instead  of  taking  one- 
fourth  of  it),  and  titrating  with  oxalic  acid  V.S.  in  the  presence  of  phenol- 
phthalein. Oxalic  acid  in  very  dilute  solutions  does  not  react  with  alkali 
earth  carbonates. 


78        THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

with  normal  acid.     5.6  mils  are  required;    then  5.6X4=22.4 
mils,  representing  the  sodium  hydroxid. 

43.2  mils  =  total  alkalinity; 
-22.4X0.040  =  0.896  gm.  sodium  hydroxid. 


20.8X0.053  =  1. 1024  gms.  sodium  carbonate. 

Another  way  is  to  filter  the  mixture  after  barium  chlorid 
has  been  added,  titrate  the  filtrate  with  normal  acid  to  find 
the  quantity  of  hydroxid,  then  dissolve  the  precipitated  barium 
carbonate  in  normal  hydrochloric  acid  in  excess,  and  retitrate 
with  normal  alkali,  thus  ascertainmg  the  amount  of  carbo- 
nate. 

When  the  alkaline  carbonate  is  present  in  very  small 
quantities  the  method  of  Lunge  may  be  employed. 

A  few  drops  of  phenacetolin  solution  are  added  to  impart 
a  scarcely  perceptible  yellow  to  the  liquid.  Normal  acid 
solution  is  then  run  in  until  a  pale  rose  tint  appears,  indicating 
that  all  the  alkaU  hydroxid  is  neutralized;  the  volume  of  acid 
is  noted,  and  the  titration  continued;  the  red  color  is  inten- 
sified, and  when  the  carbonate  is  entirely  decomposed  a 
golden-yellow  color  results. 

Considerable  practice  is  required  with  solutions  of  known 
composition  to  accustom  the  eye  to  the  changes  of  color. 

Mixed  Alkali  Bicarbonate s  and  Carbonates 

Thompson'' s  Method.  Take  2  grams  of  the  salt  and  dissolve 
in  100  mils  of  water.  Divide  the  solution  into  two  equal  parts 
and  titrate  one  portion  with  normal  acid  solution,  using  methyl 
orange  as  indicator,  and  note  the  quantity  required.  We  will 
assume  13  mils. 

Then  treat  the  second  portion  with  a  measured  excess 
(say  25  mils)  of  normal  sodium  hydroxid  solution  free  from 


ANALYSIS  BY  NEUTRALIZATION  79 

CO2.  This  converts  the  bicarbonate  into  carbonate.  Now 
add  an  excess  of  pure  neutral  barium  chlorid  solution  in 
order  to  precipitate  all  the  carbonate  as  barium  carbonate, 
and  then  titrate  with  normal  acid,  using  phenolphthalein  as 
indicator,  to  determine  the  excess  of  sodium  hydroxid.  15 
mils  are  required.     Thus 

25  -15  =  10  mils,  the  equivalent  of  bicarbonate; 
and         13  -10  =  3  mils,  the  equivalent  of  carbonate; 
10 X. 084  =  .840  gm.  sodium  bicarbonate; 
3  X  .053  =  .159  gm.  sodium  carbonate. 

Sodium  Borate  (Borax)  (Na2B407  +  10H2O  =  382.16).  Dis- 
solve 5  gms.  of  the  salt  in  100  mils  of  distilled  water  and  titrate 
the  solution  with  normal  hydrochloric  acid  V.S.,  using  methyl 
orange  T.S.  as  indicator. 

Na2B407  •  10H2O  +  2HCI  =  2NaCl  +  4H3BO3  +  5H2O. 
382.16 

Each  mil  of  normal  HCl  V.S.  corresponds  to  0.19108  gm. 
of  crystallized  borax  or  to  o.ioi  gm.  of  anhydrous  borax. 

Sodium  Cacodylate  (Na(CH3)2As02=  160.01).  This  salt 
is  assayed  by  titration  with  normal  HCl  V.S.  in  the  presence 
of  methyl  orange.  Each  mil  of  the  acid  V.S.  corresponds  to 
0.160  gm.  of  the  salt. 

This  salt  is  occasionally  slightly  acid  in  reacti'on ;  if  so,  it 
should  be  carefully  neutralized  to  phenolphthalein  before 
titration  is  begun. 

Sodiimi    Glycerophosphate    (Na2C3H5(OH)2P04  =  216.1). 

This  salt  is  titrated  with  half -normal  hydrochloric  acid  V.S. 

in  presence  of  methyl  orange. 

N 
Each  mil  of  —  acid  V.S.  corresponds  to  0.10805  gm.  oi 

the  salt. 


I 


80         THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

Estimation  of  Organic  Salts  of  the  Alkalies 

The  tartrates,  citrates  and  acetates  of  the  alkali  metals  are 
converted  by  ignition  into  carbonates,  the  whole  of  the  base 
remaining  in  the  form  of  carbonate. 

Each  molecular  weight  of  a  normal  tartrate  gives  when 
ignited  one  molecular  weight  of  carbonate: 

K2C4H406=  K2CO3. 

Every  two  molecular  weights  of  an  acetate  or  an  acid 
tartrate  give  one  molecular  weight  of  carbonate: 

2KC2H302  =  K2C03; 

2KHC4H406  =  K2C03. 

Every  two  molecular  weights  of  a  normal  citrate  give 
three  molecular  weights  of  carbonate : 

2K3C6H5O7-3K2CO3. 

These  reactions  are  taken  advantage  of  in  volumetric 
analysis,  and  the  tartrates,  citrates  and  acetates  of  the  alkalies 
are  indirectly  estimated  by  calculating  upon  the  quantity  of 
carbonate  formed  by  burning  them,  the  quantity  of  carbonate 
being  found  by  titration  in  the  usual  manner. 

The  Process.  Before  igniting,  the  salt  to  be  examined 
should  be  thoroughly  dried  in  a  desiccator  over  calcium  chlorid 
or  in  a  drying  oven,  the  latter  only  for  such  salts  as  have  no 
water  of  crystallization  in  their  composition.  If  the  weight 
is  taken  before  and  after,  the  amount  of  moisture  present  is 
determined.  One  or  two  grams  of  the  dried  salt  is  weighed 
accurately,  placed  in  a  porcelain  crucible,  and  heat  applied 
gradually,  until  dull  redness  is  reached  and  white  fumes  cease 
to  be  given  off.  Upon  applying  heat  to  the  salt,  the  latter 
swells,  fuses,  and  then  boils,  and  if  the  heat  is  applied  too 
rapidly  at  this  point,  there  is  apt  to  be  a  considerable  loss 


ANALYSIS  BY  NEUTRALIZATION  31 

of  material  through  sputtering.  The  flame  of  the  burner 
must  not  come  'it  contact  with  the  carbonized  mass.  The 
completion  of  the  ignition  is  known  to  be  reached  when  the 
black  contents  of  the  crucible  is  dry  and  crisp.  The  crucible 
is  then  allowed  to  cool,  its  contents  disintegrated  with  the 
aid  of  a  stout  glass  rod  and  then  treated  with  boiling  water  to 
dissolve  out  the  alkali  carbonate,  and  the  solution  filtered 
through  a  small,  wetted  filter  into  a  flask  or  beaker.  The 
filtrate  should  be  perfectly  colorless.  ^If  it  has  a  yellow  or 
brownish  color  it  indicates  incomplete  ignition  and  should  be 
rejected,  and  a  fresh  quantity  of  the  salt  subjected  to  ignition. 
The  contents  of  the  crucible  and  the  filter  should  be  washed 
with  several  small  portions  of  water  until  the  washings  no 
longer  show  an  alkaline  reaction.  The  filtrate  mixed  with  the 
wash  water  is  now  titrated  with  standard  sulphuric  or  hydro- 
chloric acid,  using  methyl  orange  as  the  indicator.  From  the 
quantity  of  carbonate  found  in  the  filtrate,  the  equivalent 
amount  of  the  organic  salt  may  be  calculated.  The  quantity 
of  standard  acid  employed  is  multiplied  direct  by  the  factor 
for  the  original  salt.  The  residual  titration  method,  using  an 
excess  of  half-normal  sulphuric  acid  V.S.,  boiling,  and  retitrating 
with  half -normal  potassium  hydroxid  V.S.,  is  recommended 
in  the  U.S.P. 

In  tne  case  of  organic  salts  of  the  alkali  earths,*  residual 
titration  should  always  be  resorted  to.  The  residue  in  the 
crucible  being  dissolved  in  standard  hydrochloric  acid,  and 
retitrated  with  standard  alkali. 

Lithium  salts,  because  of  the  sparing  solubility  of  the 
carbonate  in  water,  should  also  be  titrated  by  the  residual 
method. 

Potassium  Tartrate  (K2C4H406  =  226.23).    Two  grams  of 


I 


*  Organic  salts  of  the  alkali  earths  subjected  to  ignition  as  above  are 
reduced  partly  to  oxids. 


82        THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

the  salt  is  placed  in  a  platinum  or  porcelain  crucible  and  heated 
to  redness  in  contact  with  the  air  until  completely  charred ; 
that  is  to  say,  until  nothing  is  left  in  the  crucible  but  carbonate 
and  free  carbon. 

The  crucible  is  now  cooled,  and  its.  contents  treated  with 
boiling  water,  which  dissolves  the  potassium  carbonate,  the 
carbon  being  separated  by  filtration.  In  order  to  obtain 
every  trace  of  carbonate  it  is  well  to  wash  the  crucible  with 
several  small  portions  of  hot  water,  and  add  the  washings 
to  the  rest  of  the  filtrate  through  the  filter. 

If  the  salt  is  completely  carbonized  the  filtrate  will  be 
colorless,  but  if  the  carbonization  is  not  complete  the  solution 
will  be  more  or  less  colored  and  should  be  rejected,  and  a 
fresh  quantity  of  the  salt  subjected  to  ignition. 

To  the  filtrate,  which  contains  potassium  carbonate,  add 

N 
a  few  drops  of  methyl  orange,  and  titrate  with  —  sulphuric 

acid  V.S.  until  a  light  orange-red  color  appears  and  the  car- 
bonate is  neutralized.  - 

The  following  equations  will  explain  the  reactions: 


K2G4H4O6  =  K2CO3  +  C2  +  CO  +  2H2O 

226.2  is8.2 


then 


K2CO3  +  H2SO4  =  K2SO4  +  H2O  +  CO2 

138.2  98.07 

therefore 

K2C4H4O6  =  K2CO3  =  H2SO4 

2)226.2  2)1.^8.2  2)98.07^  ^ 

113. 1  gms.     =      69.1  gms.  =  49.03  gms.  =  iooo  mils  —  V»S. 

N 
and  each  mil  of  —  H2SO4  represents  0.1131  gm.  of  potassium 

tartrate. 

Example.     Two  grams  of  potassium  treated  as  described 


ANALYSIS  BY  NEUTRALIZATION  83 


*  N 

above  require  16.3  mils  of  —  H2SO4.     It  therefore  contains 

1.1131X16.3=1.8435  gms. 


1.8435 X 100 

=  92.17  per  cent. 


f 

^m       Potassium    and     Sodium    Tartrate     (KNaC4H406.4H20 

^^■=282.22)  {Rochelle  Salt).     This  salt  is  treated  in  exactly  the 
same  way  as  described  for  potassium  tartrate. 

When  ignited  the  double  tartrate  is  converted  into  a  double 
carbonate  of  potassium  and  sodium: 

KNaC4H406  =  KNaCOs  +  etc.; 
210,1  122. 1 

then 

KNaCOa   +  H2SO4  =   KNaS04  +   CO2   +  H2O 
therefore 

KNaC4K406  =    KNaCOs  =  H2SO4 

2)210.1  2)122.1  2)98.07  ir 

105.05  61.05    .  49.03  =  1000  mils  —  V.S. 

N 
and    each    mil    of    —    H2SO4    represents    0.10505    gm.    of 

KNaC4H406. 

Example.    If  one  gram  of  rochelle  salt  treated  as  above 

N 
escribed  requires   7   mils  of  —  sulphuric  acid,    it  contains 

0.10505  X 7  =0.7353  gm.  =  73.53  per  cent. 

According  to  the  U.S. P.  IX  this  salt  is  assayed  by  residual 
titration,  using  half-normal  sulphuric  acid  V.S.,  as  follows: 

Heat  two  gms.  of  the  salt  in  a  porcelain  crucible,  until 
thoroughly  carbonized.  Allow  the  carbonized  mass  to  cool,  dis- 
integrate it  with  the  aid  of  a  stout  glass  rod  and  transfer  the  mass 
and  crucible  to  a  beaker.  Add  50  mils  of  distilled  water  and  50 
mils   of   half-normal   sulphuric   acid   V.S.,    cover   the   beaker 


k 


84       THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

with  a  watch  glass  and  boil  the  contents  for  thirty  minutes. 
Then  filter  the  solution  and  wash  the  residue  with  hot  distilled 
water  until  the  washings  cease  to  redden  blue  litmus  paper. 
Now  determine  the  residual  acid  in  the  cooled  filtrate  by  titra- 
tion with  half -normal  KOH  V.S.,  using  methyl  orange  as 
indicator. 

Subtract  the  number  of  mils  of  half-normal  alkali  V.S. 
used  from  50  mils  (the  quantity  of  half-normal  acid  V.S. 
taken).  The  remainder  represents  the  quantity  of  half -normal 
sulphuric  acid  V.S.  which  reacted  with  the  carbonate  in  the 

N 
charred   mass.     This,    multiplied   by   —    factor    for  •  rochelle 

salt,  represents  the  weight  of  the  latter  in  the  amount  taken 
for  analysis.     The  foregoing  equations  show  the  normal  factor 
for  anhydrous  rochelle  salt  to  be  0.10505  gm.,  hence  the  half 
normal  factor  for  the  same  is  0.05253  gm. 

Hence,  if  in  the  above  assay  22  mils  of  half-normal  KOH 
V.S.  were  used,  this  quantity  deducted  from  50  mils,  leaves 
28  mils,  the  quantity  of  half- normal  sulphuric  acid  which  was 
taken  up  by  the  alkaline  carbonized  mass;  then  0.05253X28 
=  1.470  gm.  of  KNaC4H406  in  the  2  gms.  of' rochelle  salt 
taken  for  analysis  =  73.5  per  cent. 

Potassium  Bitartrate  (KHC4H4O6 ^188.1)  {Cream  of  Tar- 
tar). The  estimation  of  this  salt  is  affected  in  the  same  way 
as  the  tartrate. 

The  bitartrate  having  but  one  atom  of  potassium  in  its 
molecule,  it  takes  two  molecules  to  form  one  molecule  of  car- 
bonate. 

2KHC4H4O6  =  K2CO3  +  H2SO4 

2)376.2  2)98.07  j^ 

188.1  gms.  49.03  gms.  =  1000  mils  —  V.S. 

Each  mil  of  -  H2SO4  V.S.  =0.1881  gm.  of  KHC4H4O6. 


ANALYSIS  BY  NEUTRALIZATION  85 

Another  way  of  estimating  bitartrate  is  to  dissolve  a  weighed 

N 
quantity  of  hot  water  and  titrate  with  —  potassium   hydroxid 

until  neutral,  and  thus  the  amount  of  tartaric  acid  existing 
as  bitartrate  is  found.  The  bitartrate  is  acid  in  reaction- 
In  detail  the  method  is  as  follows : 

Two  grams  of  the  bitartrate  are  dissolved  in  loo  mils  of 
hot  water,  a  few  drops  of  phenolphthalein  T.S.  added,  and 

N 
then  titrated  with  —  KOH  V.S.  until  a  faint  pink  color  indi- 
cates that  all  of  the  acid  has  been  neutralized.     Not  less  than 
I0.6  mils  of  the  normal  alakli  should  be  required,  corresponding 
to  99.6  per  cent  of  pure  salt. 

The  following  equation  will  show  the  reaction: 

KHC4H4O6  -f   KOH  =  K2C4H4O6  +  H2O. 

188.1  56.1      =  1000  mils  of  -  KOH  V.S. 

I 

N 
Each  mil  of  —    KOH    V.S.     represents    0.1881     gm.    of 

KHC4H4O6. 

If  10.6  mils  are  required  for  neutralization,  then  io.6Xo.i88i 

=  1-993+  gnis.: 

1.993  X 100 


2 


=  99.6  per  cent. 


Potassium  Citrate  (K3C6H507  =  306.3). 

2K3C6H5O7  =  3K2CO3  =  3H2SO4. 

6)612.6  6)414.6  6)294.18  '  J- 

102.1  gms.  69.1       =  49.03  gms.  =  ICXX3  mils  —  acid. 

N 
Thus  each  mil  of  —  acid  represents  0.1021  gm.  of  pure 

N 
potassium  citrate,  and  each  mil  of  —  acid  represents  0.0510  gm. 

Potassium  Acetate  (KC2H302  =  98.i).     In  estimating  potas- 
sium acetate  the  salt  is  ignited  and  the  residue  treated  in 


86        THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

exactly  the  same  manner  as  in  the  estimation  of  the  citrates 
and  tartrates  before  mentioned. 

2KC2H3O2  =  K2CO3. 

2)196-2  j^ 

98.1  gms.  =  1000  mils  —  H2SO4. 

N 
Each  mil  therefore  of  —  H2SO4  corresponds  to  0.0981 

gm.  of  potassium  acetate. 

Sodium  Acetate  (NaC2H302 .  3H2O  =  136.09) . 
2  (NaC2H302 .  3H2O)  =  NasCOs, 

N 
Each  mil  of  —  H2SO4  V.S.  represents  0.06804  gm.  of  crys- 
tallized sodium  acetate. 

Sodium  Benzoate  (NaC7H502=  144.05). 

2NaC7H502  =  Na2C03. 

N 
Each  mil  of  —  H2SO4  V.S.  represents  0.07202  gm.  of  sodium 

benzoate. 

Sodium  Salicylate  (NaC7H503=  160.05). 

2NaC7H503  =  Na2C03. 

N 
Each  mil  of  —  H2SO4  V.S.  represents  0.08002  gm.  of  sodium 

salicylate. 

Lithiimi  Citrate  (Li3C6H507  =  209.92).  As  stated  before, 
the  organic  salts  of  lithium  and  those  of  the  alkali  earth 
metals  are  best  titrated  by  the  residual  method,  after  ignition, 
because  the  carbonates  formed  are  insoluble  in  water.  It  is 
likewise  best  to  use  standard  hydrochloric  instead  of  standard 
sulphuric  acid.  The  process  for  lithium  citrate  here  given 
exemplifies  the  method. 

Two  grams  of  the  salt  is  thoroughly  ignited  in  a  porcelain 
crucible  as  described  for  potassium  tartrate.  The  residue  of 
lithium  carbonate  is  then  dissolved  out  of  the  crucible  by  add- 


I 


ANALYSIS  BY  NEUTRALIZATION  87 


N 
ing  50  mils  of  —  hydrochloric  V.S.  and  filtering.     The  crucible 

and  filter  are  washed  with  several  small  quantities  of  water 
and  the  washings  added  to  the  acid  filtrate.  Three  drops  of 
methyl  orange  are  now  added,  and  the  solution  titrated  with 

N 

^  potassium  hydroxid  V.S.  until  the  yellow  color  appears. 

Assuming  that  8  mils  of  the  standard  alkali  were  required, 
then  50-8  =  42  mils,  the  quantity  of  half-normal  hydrochloric 
acid  which  reacted  with  the  lithium  carbonate.  This  quantity 
multiplied  by  the  half -normal  factor  for  lithium  citrate,  0.03498, 
gives  the  weight  of  pure  salt  in  the  2  gms.  taken. 

0.03498X42  =  1. 473 1  +  gm.  or  73.65  per  cent. 

Calcium  Lactate  (Ca(C3H503)2  +  5H20  =  308.23).  This  is 
assayed  like  the  foregoing,  half  normal  hydrochloric  acid  V.S. 

Each  mil  of  half  normal  acid  V.S.  corresponds  to  0.05454 
gm.  of  Ca(C3H503)2. 

Strontium  Salicylate  (Sr(C7H503)2  + 21120  =  397.74).  This 
is  assayed  like  the  foregoing,  but  care  must  be  taken  not  to 
allow  the  temperature  to  exceed  red  heat  during  the  carbon- 
ization. Each  mil  of  half  normal  hydrochloric  acid  V.S.  cor- 
responds to  0.099435  gm.  of  Sr(C7H503)2  +  2H20. 

Pulvis  Effervescens  Compositus  {Seidlitz  Powder).  The 
weight  of  the  mixture  in  the  blue  paper  should  be  not  less 
than  9.5  gms.  nor  more  than  10.5  gms.  It  should  contain 
not  less  than  23  per  cent  nor  more  than  27  per  cent  of  sodium 
bicarbonate,  and  not  less  than  73  per  cent  nor  more  than  78 
per  cent  of  rochelle  salt. 

The  Assay  for  Sodium  Bicarbonate.  Dissolve  2  gms.  of 
the  contents  of  the  blue  paper  in  80  mils  of  distilled  water, 
add  20  mils  of  half-normal  sulphuric  acid  V.S.,  boil  the  solu- 
tion until  the  volume  is  reduced  to  about  50  mils  and  titrate 
the  excess  of  acid  with  half-normal  potassium  hydroxid  V.S., 


88       THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

using  phenolphthalein  T.S.  as  indicator.  Subtract  the  number 
of  mils  of  half-normal  potassium  hydroxid  V.S.  used  from  20 
mils  (the  quantity  of  half-normal  sulphuric  acid  taken)  and 
the  remainder  will  be  the  quantity  of  the  latter  which  reacted 
with  the  sodium  bicarbonate. 
The  calculation  is  as  follows: 

2NaHC03  +  H2SO4  =  Na2S04  +  2H2O  +  2CO2. 

2)168.02  2)98.09 

2)  84.01  2)49.045 

42.005  24.522  gms.  in  1000  mils  of  half-normal  V.S. 

Each  mil  of  half-normal  acid  V.S.  =  0.042005  gm.  of 
NaHCOa. 

Assuming  that  8  mils  of  half-normal  potassium  hydroxid 
were  consumed  in  the  assay  then  12  mils  of  half-normal  sul- 
phuric acid  reacted  with,  and  hence  represent  the  sodium 
bicarbonate  present. 

Hence  0.042X12  =  0.504  gm.  or  25.2  per  cent. 

The  Assay]  for  Rochelle  Salt.  Take  2  gms.  of  the  contents 
of  the  blue  paper  used  in  the  preceding  assay,  place  it  in  a 
porcelain  crucible  and  carbonize  it  as  described  under  potas- 
sium and  sodium  tartrate.  The  difference  between  the  num- 
ber of  mils  of  half-normal  sulphuric  acid  V.S.  consumed 
in  this  assay  and  in  the  preceding  assay  for  sodium  bicarbonate 
multiplied  by  0.07055  represents  the  rochelle  salt. 

N 
Thus,  50 -o  mils  of  —  H2SO4  V.S.  taken 

N 
16.5  mils  of  —  KOH  V.S.  required  for  neutralization 

N 
33.5  mils  of  —  H2SO4  consumed 

33.5  mils  -12  mils  =  21.5  mils,  representing  rochelle  salt; 
0.07055X21.5  =  1.516  gm.  or  75.8  per  cent  of  crystallized 
rochelle  salt. 


ANALYSIS  BY  NEUTRALIZATION  89 

Ammonium  Benzoate  (NH4C7H502=  139.08)  and  Ammo- 
niimi  Salicylate  (NH4C7H503=  155.08)-  It  is  evident  that 
because  of  the  volatiHty  of  ammonia,  these  salts  cannot  be 
assayed  by  the  method  employed  in  the  case  of  organic  salts 
of  the  fixed  alkalies.  The  official  assays  of  these  salts  depends 
upon  the  liberation  of  the  organic  acid,  by  the  addition  of 
sulphuric  acid,  and  its  extraction  with  an  inamiscible  solvent. 
The  solution  so  obtained  is  evaporated  and  the  organic  acid 
residue  dissolved  in  neutralized  diluted  alcohol  and  titrated 
with  tenth-normal  barium  hydroxid  V.S.  The  method  in 
detail  is  as  follows:  Dissolve  about  0.5  gm.  of  the  salt  pre- 
viously dried  in  a  desiccator  over  sulphuric  acid,  and  accurately 
weighed,  in  10  mils  of  distilled  water,  in  a  separator.  Add 
to'  the  solution  5  mils  of  diluted  sulphuric  acid  and  extract 
the  liberated  organic  acid  by  shaking  out  with  three  successive 
portions  of  25,  15,  and  10  mils  respectively  of  chloroform, 
passing  the  chloroform  solution  through  a  filter  previously 
moistened  with  chloroform,  and  removing  any  of  the  organic 
acid  adhering  to  the  stem  of  the  funnel  with  a  few  mils  of 
chloroform.  Evaporate  the  chloroform  solution  at  a  very 
low  temperature  to  5  mils,  add  25  mils  of  diluted  alcohol, 

N 
which  has  been  previously  neutralized  with  —  KOH  V.S.  in 

the  presence  of  phenolphthalein. 

N        .  . 

Titrate  this  solution  with  —  barium  hydroxid  V.S.,  using 

phenolphthalein  as  indicator. 

N 
Each  mil  of  the  —  Ba(0H)2  V.S.  represents  benzoic  acid, 

0.012205'gm.;  salicylic  acid,  0.013805  gm.;  ammonium  benzoate 
0.013908  gm.,  and  ammonium  salicylate  0.015508  gm. 


k 


90        THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

TABLE  SHOWING  THE  NORMAL  FACTORS,  ETC.,  OF  THE 
ORGANIC   SALTS  OF  THE  ALKALI  METALS. 


Substance. 


Lithium  benzoate 

'  *        citrate 

"        salicylate 

Sodium  acetate 

* '       benzoate 

*  *       salicylate 

Potassium  acetate 

* '  bitartrate. .  .  . 

"  citrate 

"         tartrate 

"         and  sodium 
tartrate. .  . . 


Formula. 


LiCvHsOz 

LisCeHsOy 

LiCvH^Os 

NaCsHsOa.sHaO 

NaCyHeOz 

NaC7H503 

KC2H3O2 

KHC4H4O6 

K3C6H5O7.H2O 

K2C4H4O6IH2O 

KNaC4H406.4H20 


Molecular 
Weight. 


127.98 
209.92 
143-98 
136.07 
144.04 
160.04 
98.12 
188.14 
32436 
235-24 

282. 20 


Equivalent 
Weight  in 
Carbonate. 


36.94 
92-35 
36-94 
53-0 
53-0 
53-0 
69. 1 
69.  I 
207.3 
138.2 


Normal 
Factor. 


o. 12798 
0.06997 


14398 
13607 
14404 
16004 
09812 
1 88 1 4 
10812 
11762 


o. 141] 


Estimation  of  the  Salts  of  the  Alkali  Earths 

Standard  solution  of  hydrochloric  or  of  nitric  acid  is  pre- 
ferred by  many  operators  for  the  titration  of  hydroxids  or 
carbonates  of  the  alkah  earths.  These  acids  possess  the 
advantage  over  most  other  acids  of  forming  soluble  salts. 
The  hydroxids  may  be  estimated  by  any  of  the  indicators, 
but  as  they  readily  absorb  CO 2  out  of  the  air  they  are  generally 
contaminated  with  more  or  less  carbonate,  and  the  residual 
method  should  be  used,  i.e.,  a  known  excess  of  standard  acid 
should  be  added,  the  mixture  boiled  to  expel  any  trace  of  CO2, 
and  titrated  with  standard  alkali. 

The  carbonates  are  of  course  estimated  in  the  same  way, 
as  are  also  the  organic  salts  of  the  alkali  earths,  after  ignition. 
As  an  example : 

One  gram  of  calcium  carbonate  is  mixed  wnth  5  mils 
of  water.  An  excess  of  normal  hydrochloric  acid  V.S.  is 
now  added,  and  the  solution  boiled  to  drive  oEf  the  CO2. 
Then  a  few  drops  of  phenolphthalein   T.S.   are  added,   and 


ANALYSIS  BY  NEUTRALIZATION  91 

N  . 

titrated    with   —  alkali  V.S.  until   a  faint  pink  color  is  ob- 
tained. 

N 
Note  the  quantity  of  —  alkali  used,  and  deduct  this  from 

N 
the  quantity  of  —  acid  first  added,  and  the  remainder  will 

represent  the  amount  of  acid  which  combined  with  the  calcium. 

N 
Each  mil  of  —  acid  V.S.  represents  0.05  gm.  of  CaCOs. 

CaCOs  +    2HCI  =  CaCl2  +  H2O   +  CO2. 

2)100  2)72.92  j^ 

50  gms.        36.46  gms.  or  1000  mils  —  acid  V.S. 

N 
Assuming  that  30  mils  of  —  HCl  V.S.  were  added  to  the  i 

N 
gm.  of  CaCOs,  and  that  11  mils  of  —  KOH  V.S.  were  re- 
quired to  bring  the  mixture  back  to  neutrality,  then  19  mils 

N 
of  —  HCl  were  actually  required  to  saturate  the  CaCOs. 

Therefore  0.050  X  19  =  0.950  or  95  per  cent. 

The  hydroxids  and  carbonates  may  also  be  estimated  by 
direct  titration  with  standard  hydrochloric  acid  (in  the  cold) 
using  methyl  orange  as  indicator.  A  better  plan,  however, 
would  be  to  add  the  standard  acid  in  slight  excess,  and  then 
standard  alkali  until  a  distinct  yellow  color  appears;  the' 
slight  excess  of  alkali  is  then  determined  by  adding  standard 
hydrochloric  acid  until  the  red  color  reappears.  A  much  more 
distinct  color  reaction  is  thereby  obtained.  The  quantity  of 
the  standard  alkali  used  is  deducted  from  the  total  quantity 
of  standard  acid  added. 

Soluble  salts  of  calcium,  barium  and  strontium,  such  as 
chlorids,  nitrates,  etc.,  may  be  readily  estimated  as  follows: 


92        THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

A  weighed  quantity  of  the  salt  is  dissolved  in  water, 
cautiously  neutralized  if  it  is  acid  or  alkaline,  phenolphthalein 
is  added,  the  mixture  heated  to  boiling,  and  standard  solution 
of  sodium  carbonate  delivered  in  from  time  to  time,  with 
constant  boiling  until  the  red  color  is  permanent. 

This  process  depends  upon  the  fact  that  sodium  carbonate 
forms  with  soluble  salts  of  these  bases  insoluble  neutral  car- 
bonates. 

CaCb  +  NasCOs  =  CaCOs  +  2NaCl. 
Ba(N03)2  +  NasCOs  =  BaCOs  +  2NaN03. 

As  an  example  of  the  process:    Take  of  calcium  chlorid 

one  gram,  dissolve  it  in  a  small  quantity  of  water,  neutralize 

the  solution  if  it  is  acid  or  alkaline,  heat  to  boiling,  add   a 

N 
few   drops   of  phenolphthalein,   and   titrate   with  —    sodium 

carbonate,  delivered  cautiously  while  boiling  until  the  red 
color  is  permanent. 

CaCl2  +  NasCOs  =   CaCOs   +  2NaCl. 

2)113  2)106  ^    j^ 

56.5  gms.        53  gms.  or  1000  mils  —  V.S. 

N 
Each  mil  of  —  Na2C03  V.S.  represents  0.0565    gm.  of 

CaCl2.  If  17  mils  are  used  the  salt  contains  0.0565  gm.Xiy 
=  0.96  gm.  or  96  per  cent. 

Normal  Sodium  Carbonate  V.S.  (Na2C03  =  io6)  contains 
53  gms.  in  T  liter.  This  solution  Is  made  by  dissolving  53 
gms.  of  pure  sodium  carbonate  (anhydrous)  previously  ignited 
and  cooled,  in  distilled  water,  and  diluting  to  i  liter  at  25°  C. 

If  a  puT-e  salt  is  not  at  hand  the  solution  may  be  made  as 
follows : 

About  85  gms.  of  pure  sodium  bicarbonate,  free  from 
thiosulphafe,  chlorid,  etc.,  are  heated  to  dull  redness  (not  to 


ANALYSIS  BY  NEUTRALIZATION  93 

fusion)  for  about  thirty  minutes  to  expel  one-half  of  the  CO2; 

it  is  then  cooled  under  a  desiccator.     When  cool,   53   gms. 

are  dissolved  in  distilled  water  to  make  i  liter  at  25°  C.  (77° 

N 
F.).     This   solution   should   neutralize   —   acid   V.S.    volume 
^  I 

for  volume. 

The  alkali  earths  may  also  be  estimated  by  dissolving 
them  in  water,  precipitating  the  base  as  carbonate,  with  an 
excess  of  ammonium  carbonate  and  some  free  ammonia. 
The  mixture  is  then  heated  for  a  few  minutes,  and  the  car- 
bonate separated  by  filtration,  thoroughly  washed  with  hot 
water  till  all  soluble  matters  are  removed,  and  then  titrated 
with  normal  acid  V.S.  as  directed  for  carbonate. 

CaBrs  ==   CaCOa  =  H2SO4. 

2)198-52  2)99.35  2)98  j^ 

99.26  gms.      49-675  gms.        49  gms.  or  1000  mils  y  V.S. 

N 
Each  mil  of  —  acid  thus  represents  0.09926  gm.  of  CaBr2. 

Another  method  for  the  estimation  of  salts  of  the  alkali 
earths  consists  in  precipitating  them  as  oxalates  out  of  an 
ammoniacal  or  weak  acetic  acid  solution.  The  precipitated 
oxalate  is  then,  after  thorough  washing,  titrated  with  tenth- 
normal potassium  permanganate  V.S.  Or  the  excess  of  oxalic* 
acid  in  the  filtrate  may  be  titrated  with  standard  permanganate 
V.S.  This  method  is,  however,  especially  applicable  to  calcium 
estimations  because  of  the  completeness  with  which  this  metal 
may  be  precipitated  as  oxalate. 

Magnesium  Carbonate.  One  gm.  of  magnesium  carbon- 
ate is  dissolved  in  30  mils  of  normal  sulphuric  acid  V.S.,  and 
the  excess  of  the  latter  determined  by  titration  with  normal 
potassium  hydroxid  V.S.,  using  methyl  orange  as  indicator. 
Magnesium  Oxid  and  Magnesium  Hydroxid  are  estimated  in 


94        THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

exactly  the  same  manner.  The  absence  of  calcium  oxid  must, 
however,  be  assured,  or,  if  present,  its  quantity  determined. 

This  is  done  by  precipitation  with  ammonium  oxalate  in 
the  presence  of  ammoniumi  chlorid  and  ammonium  hydroxid. 
The  precipitate  is  incinerated  and  the  residual  calcium  oxid 
weighed. 

Alkalimetric  Assay  of  Zinc  Salts.  Zinc  Oxid.  Digest  about 
1.5  gm.  of  zinc  oxid,  accurately  weighed,  with  50  mils  of  normal 
sulphuric  acid  V.S.,  until  solution  is  complete.  Then  titrate 
the  excess  of  sulphuric  acid  with  normal  potassium  hydroxid 
V.S.,  using  methyl  orange  as  indicator. 

ZnO+H2S04  =  ZnS04+H20. 

Zinc  carbonate  and  zinc  stearate  are  assayed  in  the  same 
manner.  In  the  case  of  the  latter,  boiling  for  ten  minutes 
may  be  necessary  in  order  to  completely  dissolve  the  salt. 
Each  mil  of  normal  sulphuric  acid  V.S.  corresponds  to  0.040685 
gm.  of  zinc  oxid. 

The  Estimation  of  Mixed  Hydroxids  and  Carbonates  of 
Alkali  Earths.  This  may  be  done  as  described  under  esti- 
mation of  mixed  alkali  hydroxids  and  carbonates,  page  77, 
except  that  in  this  case  it  is  unnecessary  to  precipitate  the 
carbonate  by  barium  chlorid  in  that  the  alkali  earth  carbonates 
^re  already  insoluble. 

Assay  of  Chloral  Hydrate.  Dissolve  an  accurately  weighed 
quantity  (about  4  gms;)  of  chloral  hydrate  in  10  mils  of  dis- 
tilled water,  add  30  mils  of  normal  potassium  hydroxid  V.S., 
and  let  the  solution  stand  about  two  minutes.  Then  add 
phenolphthalein  T.S.,  and  at  once  titrate  the  excess  of  alkali 
by  means  of  normal  sulphuric  acid  V.S. 

This  reaction  must  take  place  in  the  cold  or  at  least  at 
ordinary  temperature,  otherwise  the  alkali  will  attack  the 
chloroform  which  is  formed  and  yield  too  high  results.    The 


ANALYSIS  BY  NEUTRALIZATION 


95 


chloroform  liberated  produces  a  slight  turbidity  which  may 
be  dissipated  by  swinging  the  flask  for  a  minute  or  two. 
The  reaction  is  thus  expressed : 

C2HCI3O  +  H2O  +  KOH  =  CHCI3 + HCOOK  +  H2O. 

165.4 

Each  mil  of  normal  alkali  consumed  corresponds  to  0.1654 
gm.  of  C2HCI3O+H2O. 


Acidimetry 

The  Estimation  of  Acids  by  Neutralization.    In  the  preceding 
pages  it  has  been  shown  how  alkalies  are  estimated  by  the 


Fig.  35. 


Fig.  36. 


use  of  acid  solutions  of  known  neutralizing  power.     In  the 
estimation  of  acids,  which  will  now  be  described,  the  order 


96        THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

is  reversed,  alkaline  solutions  of  known  power  being  used  in 
determining  the  strength  of  acids  and  of  acid  salts.  Thus  the 
procedure  is  analogous  to  that  of  the  alkalimetric  methods. 
The  choice  of  the  indicator,  whether  litmus,  phenolphthalein, 
or  methyl  orange,  depends  upon  the  particular  acid  to  be 
estimated.  Phenolphthalein  is  employed  for  the  organic  acids 
and  boric  acid  and  is  preferred  for  phosphoric  acid;  while 
methyl  orange  and  litmus  are  usually  employed  in  the  titration 
of  the  inorganic  acids. 

The  standard  alkali  used  may  be  either  an  hydroxid  or 
a  carbonate,  the  former  is,  however,  usually  preferred,  because 
the  carbonate  when  brought  in  contact  with  an  acid  gives 
off  carbonic  acid  gas  (CO2)  which  interferes  to  a  great  extent 
with  most  indicators.  On  the  other  hand,  it  must  be  remem- 
bered that  the  alkali  hydroxids  are  very  prone  to  absorb  carbon 
dioxid  from  the  atmosphere,  therefore  their  solutions  often 
contain  some  carbonate,  the  presence  of  which  even  in  small 
quantities  will  occasion  errors  when  used  with  most  indicators, 
especially  with  litmus  and  phenolphthalein.  It  is  therefore 
advisable,  when  using  these  indicators  or  others  which  are 
affected  by  carbon  dioxid,  to  employ  gentle  heat  toward  the 
close  of  each  titration,  in  order  to  drive  off  the  liberated  gas. 
Methyl  orange  is  not  affected  by  this  gas,  and  therefore  heating 
is  not  necessary  when  this  indicator  is  used.  In  fact,  it  is 
imperative  that  heat  should  not  be  employed  with  this 
indicator. 

In  acidimetrical  operations  when  methyl  orange  is  used 
as  indicator,  residual  titrations  may  be  advantageously  done, 
because  the  change  of  color  from  yellow  to  red  which  is 
brought  about  by  the  acid  is  much  more  readily  seen  than 
that  from  red  to  yellow. 

In  the  U.  S.  P.  standard  solutions  of  both  potassium  and 
sodium  hydroxid  are  official.     The  former,  however,  is  pref- 


ANALYSIS  BY  NEUTRALIZATION  97 

erable,  because  it  attacks  glass  less  energetically,  and  also 
foams  much  less  than  does  the  sodium  hydroxid  solution. 
The  neutralizing  power  of  both  is,  however,  the  same.  Standard 
solutions  of  alkali  hydroxid  should  be  preserved  in  small  vials, 
provided  with  well-fitting  rubber  stoppers,  or  better  still,  they 
should  be  provided  with  tubes  filled  with  a  mixture  of  soda 
and  lime,  which  absorbs  CO  2  and  prevents  its  access  to  the 
solution.     A  vessel  of  this  description  is  illustrated  in  Fig.  35. 

An  improvement  upon  this  is  shown  in  Fig.  36,  since  it 
allows  of  the  burette  being  filled  without  removing  the  stopper, 
and  consequently  without  any  access  of  CO2  whatever. 

Where  a  series  of  titrations  of  the  same  kind  have  to  be 
made  with  the  same  alkali  standard  solution,  the  arrangement 
shown  in  Fig.  9  may  be  used,  both  the  reservoir  and  the 
burette  in  this  case  being  provided  with  soda-lime  tubes. 

Preparation  of  Standard  Alkali  Solutions 

Normal  Potassium  Hydroxid  V.S.  (KOH  =  56.11;   -  V.S. 

=  56.11  gms.  in  1000  mils).  Potassium  hydroxid  being  prone 
to  absorb  carbon  dioxid  out  of  the  air  the  pure  article  is  not 
readily  obtained  in  commerce.  If  pure  potassium  hydroxid 
were  easily  obtained  it  would  only  be  necessary  to  dissolve  56.1 
gms.  in  sufficient  water  to  make  1000  mils.  But  since  it  always 
contains  some  CO  2  and  H2O,  it  is  necessary  to  take  a  slight 
excess  and  dilute  the  solution  to  the  proper  volume  after  having 
determined  its  strength. 

The  standardization  may  be  effected  by  means  of  any  of 
the  standard  acid  solutions.- 

A  satisfactory  method  for  the  preparation  and  standard- 
ization of  this  solution  is  as  follows : 

Dissolve  75  gms.  of  potassium  hydroxid  in  sufficient  recently 


98        THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

boiled  distilled  water  to  make  about  1050  mils  at  25°  C.  (77° 
F.),  and  fill  a  burette  with  a  portion  of  this  solution. 

Dissolve  0.63  gm.  of  pure  oxalic  acid  in  about  10  mils  of 
water  in  a  beaker  or  flask,  add  a  few  drops  of  phenolphthalein 
T.S.,  and  then  carefully  add  from  the  burette  the  potassium 
hydroxid  solution,  agitating  frequently  and  regulating  the  flow 
to  drops  towards  the  end  of  the  operation  until  a  permanent 
pale  pink  color  is  obtained.  Note  the  number  of  mils  of  the 
alkali  solution  consumed,  and  then  dilute  the  remainder  so 
that  exactly  10  mils  of  the  diluted  liquid  will  be  required  to 
neutralize  0.63  gm.  of  oxalic  acid.  Instead  of  weighing  off 
0.63  gm.  of  the  acid,  10  mils  of  its  normal  solution  may  be 
used. 

Example.  Assuming  that  8  mils  of  the  stronger  potassium 
hydroxid  solution  had  been  consumed  in  the  trial,  then  each 
8  mils  must  be  diluted  to  10  mils,  or  the  whole  or  the  remaining 
solution  in  the  same  proportion.  Thus  if  8  mils  must  be 
diluted  to  10  mils,  icco  mils  must  be  diluted  to  1250  mils. 

8  :  10::  1000  :  X        :r=  1250  mils. 

It  is  always  advisable  to  make  another  trial  after  diluting. 
Ten  mils  should  then  neutralize  0.63  gm.  of  pure  oxaHc  acid. 
Standardization  hy  Means  of  Potassium   BitaHrate, 

This  method  is  based  upon  the  reaction 

KHC4H4O6  +  KOH  =  K2C4H4O6 + H2O. 

188.14  56.1 

N 
1000  mils  of  —  KOH  contains  56.11  gms.  of  KOH  and 

will  react  with  188.14  gms.  of  potassium  bitartrate.     25  mils 

N 
of  —  KOH  will  therefore  react  with  4.7035  gms.  of  potassium 

bitartrate. 

A  solution  of  potassium  hydroxid,  75  gms.  in  1050  mils,  is 


ANALYSIS  BY  NEUTRALIZATION  99 

prepared    and    titrated    against    pure    potassium    bitartrate, 
using  phenolphthalein  as  indicator. 

Into  a  flask  of  about  300  mils  capacity  introduce  4.7035 
gms.  of  purified  dry  potassium  bitartrate,*  followed  by  15 
mils  (accurately  measured  at  25°  C.)  of  the  potassium  hydroxid 
solution  which  is  being  prepared  and  80  mils  of  distilled  water. 
Heat  the  solution  to  boiling,  add  from  3  to  5  drops  of  phenol- 
phthalein T.S.  and  then  cautiously  add  from  a  burette  further 
portions  of  the  potassium  hydroxid  solution.  Agitate  the  flask 
frequently,  boil  the  liquid  toward  the  end  of  the  operation  and 
reduce  the  flow  of  the  potassium  hydroxid  solution  to  drops 
until  the  red  color  produced  by  its  influx  no  longer  disappears 
on  shaking  and  the  liquid  is  not  deeper  in  color  than  a  pale 
pink.  Note  the  number  of  mils  of  the  potassium  hydroxid 
solution  consumed  and  then  dilute  the  remainder  of  the 
solution  so  that  exactly  25  mils  of  the  diluted  liquid  at 
standard  temperature  shall  be  required  to  neutralize  the 
4.7035  gms.  of  potassium  bitartrate.  If  only  22  mils  are 
consumed  in  the  trial,. then  each  22  mils  must  be  diluted  to 
25  mils  or  the  whole  of  the  remaining  solution  in  the  same 
proportion. 

*  Purified  potassium  bitartrate  for  standardizing  caustic  alkali  volumetric 
solutions  may  be  obtained  as  follows:  100  gms.  of  the  salt  are  placed  in  a 
beaker,  together  with  85  mils  of  water  and  25  mils  of  10  per  cent  hydrochloric 
acid,  the  beaker  is  covered  and  heated  on  a  boiling  water-bath,  stirring 
occasionally  for  three  hours.  The  liquid  is  then  quickly  cooled,  decanted, 
and  the  residue  washed  first  by  decantation  with  100  mils  of  cold  water,  then 
again  washed  after  it  has  been  transferred  to  a  plain  filter,  using  cold  water 
until  the  filtrate  ceases  to  become  opalescent  when  acidified  with  a  few  drops 
of  nitric  acid  upon  the  addition  of  silver  nitrate  solution.  The  precipitate  is 
then  dissolved  in  the  smallest  possible  quantity  of  boiling  water  (about  1500 
mils)  filtered,  and  the  filtrate  stirred  constantly  while  cooling.  When  cold, 
the  crystalline  precipitate  is.  collected  upon  a  filter  washed  with  300  mils  of 
cold  water,  run  through  it  in  small  portions  at  a  time.  It  is  then  allowed 
to  drain,  and  finally  dried  in  an  air  oven  at  j  20°  C.  until  its  weight  is  constant. 


100     THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

Standardization   hy   Means   of  JPotassiuni   Bi-iodate,* 

Potassium  bi-iodate  is  an  acid  salt  and  may  be  directly 
titrated  with  potassium  hydroxid,  using  phenolphthalein  as 
indicator.        • 

One  molecule  of  the  bi-iodate  is  equivalent  to  one  molecule 
of  potassium  hydroxid,  as  shown  by  the  equation, 

KH  (103)2  +  KOH  =  2KIO3  +  H2O. 

389.94  56.11 

To  standardize  a  potassium-hydroxid  solution,  weigh  accu- 
rately 3.8994  gms.  of  potassium  bi-iodate,  dissolve  it  in  about 
25  mils  of  water,  add  a  few  drops  of  phejiolphthalein,  and  then 
run  into  this,  from  a  burette,  the  hydroxid  solution  which  is 
to  be  standardized,  until  a  pale  pink  color  appears.  Note 
the  number  of  mils  used  and  dilute  the  solution  so  that  exactly 
10  mils  of  it  will  neutralize  3.8994  gms.  of  the  bi-iodate. 

Example.  Assuming  that  ^.2  mils  had  been  consumed, 
then  each  8.2  mils  must  be  diluted  to  10  mils,  or  the  whole 
of  the  remaining  solution  in  the  same  proportion. 

The  advantages  of  this  salt  as  an  ultimate  standard  are 
(i)  that  it  may  be  procured  in  the  market  in  a  state  of  absolute 
purity;  t  (2)  that  it  is  permanent,  being  neither  deliquescent 
nor  efflorescent;  (3)  that  it  can  be  dried  at  110°  C.  without 
decomposition;  (4)  that  the  results  obtained  with  it  are  quite 
accurate,  and  (5)  that  it  may  be  employed  for  standardizing 
most  of  the  volumetric  solutions  commonly  found  in  the 
laboratory. 

*See  Meinecke,  Chem.  Ztg.,  XIX.  2.  Also  Caspari,  Proc.  A.  Ph.  A., 
1904,  389. 

t  According  to  Caspari,  the  salt  may  be  readily  prepared  as  follows:  Se® 
A.  Ph.  A.,  1904,  390.  Potassium  bicarbonate  is  mixed  in  solution  with  an 
equivalent  amount  of  iodic  acid,  and  to  the  neutral  solution  is  added  an 
amount  of  iodic  acid  equal  to  the  quantity  first  used.  The  solution  is  then 
evaporated  until  crystallization  begins,  and  the  first  crop  of  crystals  rejected. 
Those  which  separate  after  the  solution  has  cooled  to  50°  C.  are  almost  pure 
and  will  be  rendered  absolutely  purg  if  recrystaUized.. 


ANALYSIS   BY  NEUTRALIZATION  101 

Standardizatiofi  by  Means  of  Normal  Acid  V,S.  20  mils 
of  a  strictly  normal  add  V.S.  are  placed  into  a  beaker,  2 
drops  of  methyl  orange  T.S.  are  added  and  the  potassium 
hydroxid  solution  delivered  into  it  until  the  red  liquid  just 
turns  yellow.  If  the  alkali  hydroxid  solution  is  strictly  normal, 
there  will  be  consumed  exactly  20  mils.  If  less  is  consumed 
the  solution  is  too  strong  and  must  be  so  diluted  with  distilled 
water  that  equal  volumes  of  it  and  the  normal  acid  will  exactly 
neutralize  each  other.  Thus  if  18  mils  of  the  alkali  are  con- 
sumed, then  each  18  mils  must  be  diluted  to  20  mils. 

N 
Normal  Sodium  Hydroxid  V.S.  (NaOH  =  4o;    —  V.S.  =  40 

gms.  in  1000  mils).  Dissolve  54«gms.  of  sodium  hydroxid  in 
enough  recently  boiled  distilled  water  to  make  about  1050 
mils  of  solution,  fill  a  burette  with  a  portion  of  this,  an(^  cnecK 
it  with  normal  acid,  or  a  weighed  quantity  of  oxalic  acid  or 
potassium  bitartrate,  in  exactly  the  same  manner  as  described 
for  normal  potassium  hydroxid. 

Other  strengths  of  standard  alkali  V.S.   are  half-normal 

(-),    Fifth-normal,    (-),    Tenth-normal    (—  L    Twentieth- 

/N\  /N\  /N\ 

normal  ( —    ,  Fiftieth-normal    —    ,  Hundredth-normal  ( —  ) . 
\2o/  V50/'      ^     ^  VW 

These  are  all  prepared  by  properly  diluting  the  normal  V.S. 

and  then  checking  the  strength  of  the  product. 

Other  standard  alkali  solutions  in  frequent  use  are  normal 
sodium  carbonate,  normal  and  other  strengths  of  ammonia, 
and  tenth-normal  barium  hydroxid. 

Tenth  Normal  Barium  Hydroxid  V.S.  (Ba(OH)2  +  8H20  = 

AT 

315.51  —  V.S.  =  15.776  gms.  in  1000  mils).     Dissolve  about  18 

gms.  of  crystallized  barium  hydroxid  in  1000  mils  of  recently 
boiled  distilled  water,  and,  if  necessary,  filter  the  solution 
quickly. 


102      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

Introduce  25  mils  of  tenth-normal  hydrochloric  acid  V.S. 
into  a  flask,  add  3  drops  of  phenolphthalein  T.S.  and  run  'in 
the  barium  hydroxid  solution  from  a  burette,  provided  with 
a  soda-lime  tube,  reducing  the  flow  to  drops  toward  the  end 
until  a  pale  pink  color  is  obtained  which  does  not  disappear 
on  shaking  the  liquid  for  ten  seconds.  Note  the  number  of 
mils  consumed  and  if  the  solution  is  too  concentrated,  dilute  the 
remainder  with  freshly  boiled  and  cooled  distilled  water,  so  that 
equal  volumes  of  the  barium  hydroxid  solution  and  of  tenth-nor- 
mal hydrochloric  acid  V.S.  at  25°  C.  exactly  neutralize  each  other. 

Note.  This  solution  absorbs  CO2  from  the  air  very  rapidly, 
and  thereby  loses  its  titer.  It  should  therefore  be  preserved 
in  rubber-stoppered  bottles  provided  with  a  soda-lime  tube. 
(See  Fig.  35.)  This  solution  must  be  standardized  each  time 
before  using. 

Estimation  of  the  Inorganic  Acids 

To  weigh  off  directly  a  definite  quantity  of  a  fluid  acid,  is 
not  a  very  easy  matter.  It  is  always  a  better  plan  to  measure 
a  small  quantity  of  the  acid  and  weigh  it  accurately  in  a  tared 
and  stoppered  weighing  flask  (Fig.  37),  then  to  add 
water  and  titrate  with  the  standard  alkali  .  in  the 
presence  of  a  suitable  indicator.  If  the  specific  gravity 
of  the  acid  is  known  or  can  be  easily  taken,  it  is 
sufficient  to  measure  a  certain  quantity  of  it  by 
P^  means  of  a  pipette,  and  then  determine  its  weight 
by  multiplying  the  volume  in  cubic  centimeters  by 
the  specific  gravity.  It  must  be  remembered,  however,  that 
the  liquid  must  be  measured  at  the  same  temperature  at 
which  the  specific  gravity  was  taken.  This  method  is 
applicable  to  the  diluted  acids  as  well  as  to  the  concentrated 
acids  of  commerce,  as  hydrochloric,  nitric  and  sulphuric. 


ANALYSIS  BY  NEUTRALIZATION 


103 


In  the  case  of  very  volatile  acids,  i.e.,  such  as  evolve  acid 
vapors  at  ordinary  temperatures,  the  determination  of  the 
weight  by  means  of  the  specific  gravity  is  inadmissable.  Such 
acids  should  be  weighed  in  a  Lunge  pipette.  Fig.  ^8,  or  in  a 
simple  bulb  pipette  provided  with  a  glass  stop-cock,  Fig,  39, 
or  in  a  Grethan's  pipette.  Fig.  40. 

The  Lunge  pipette  is  used  by  producing  a  vacuum  in 
the  bulb  (a),  the  air-tight  glass  mantle  (c)  is  then  removed, 


Fig.  38. 


Fig.  39. 


Fig.  40. 


and  the  tip  of  the  tube  (d)  sunk  into  the  acid  which  is  drawn 
up  into  the  bulb,  upon  opening  the  cock  (b);  when  sufficient 
of  the  acid  has  been  drawn  into  the  apparatus  the  cock  is 
closed,  the  tip  of  the  pipette  wiped,  the  glass  mantle  put  in 
place,  and  the  whole  weighed.  The  weight  of  the  empty 
pipette  deducted  gives  the  weight  of  the  acid  taken  up.  The 
pipettes  shown  in  Figs.  39  and  40  are  filled  by  applying  direct 
suction  with  the  lips,  the  operator  protecting  himself  against 
inhalation  of  harmful  vapors  by  attaching  an  absorption  tube 
containing  soda-lime,  caustic  soda,  or  similar  substance. 


104      THE   ESSENTIALS   OF   VOLUMETRIC   ANALYSIS 

The  quantity  of  acid  to  be  taken  (in  most  cases)  should 
be  such  as  will  require  for  neutralization  from  20  to  50  mils 
of  the  standard  alkali.  In  the  case  of  concentrated  inorganic 
acids,  2  or  3  gms.  may  be  taken,  while  in  the  case  of  the  dilute 
acids,  from  6  to  8  gms. 

Any  of  the  indicators  may  be  employed  for  the  inorganic 
acids,  but  because  of  the  usual  presence  of  carbonate  in  the 
standard  alkali,  methyl  orange  is  preferred. 

Hydrochloric  Acid  (1101  =  36.47).  About  2  mils  of  hydro- 
chloric acid  (sp.gr.  1.155)  are  introduced  into  a  tared  weighing 
flask  and  its  weight  accurately  taken.  (The  weight  is  found 
to  be  2.098  gms.)  Fifty  mils  of  water  are  now  added,  followed 
by  2  drops  of  methyl  orange,  and  the  solution  carefully  titrated 
with  normal  potassium  hydroxid  until  the  reddish  color  of 
the  solution  is  changed  to  yellow. 

Assuming  that  18.4  mils  were  required,  then  18.4  milsX 
0.03647  gm.  =0.671  gm.  of  absolute  hydrochloric  acid  in  the 
2.098  gms.  taken. 

To  find  the  per  cent  apply  the  proportion    . 

2.098  gms.  :  0.671  gm.::  100  :  x,        ^^  =  31.9  per  cent. 
The  equation  is : 

HCl   +  KOH  =  KCl  +  H2O. 

N 
36.47  gms.  =  56.1  gms.  =  1000  mils  —  V.S. 

N 
.03647  gm.  =       I  mil  —  V.S. 

Sulphuric  Acid  (H2S04  =  98.09).  About  i  mil  of  the  con- 
centrated acid  is  weighed  in  a  tared  weighing  flask  and  found 
to  weigh  1.8  gms.  Fifty  jnils  of  water  are  added  and  then 
2  drops  of  methyl  orange,  and  the  titration  with  normal  potas- 
sium   hydroxid    begun,    and    cautiously    continued    until    the 


ANALYSIS  BY  NEUTRALIZATION  105 

yellowish  color  of  the  solution  indicates  the  completion  of 
the  operation.  Note  the  number  of  mils  of  alkali  solution  used 
and  apply  the  equation 

H2SO4  +  2KOH  =  K2SO4  4-  2H2O. 

2)98.09    2)112.2  j^ 

49.04  gms.  =     56.1  gms.  =  1000  mils  —  V.S. 

Thus  each  mil  of  normal  KOH  V.S.  represents  0.04904 
gm.  of  pure  H2SO4. 

Aromatic  Sulphuric  Acid.  This  is  an  alcoholic  solution 
containing  free  sulphuric  acid  and  ethyl  sulphuric  together 
with  a  volatile  dil.  The  latter  as  well  as  the  alcohol  present 
neutralizes  alkali  hydroxid,  and  must  therefore  be  removed 
by  heating  before  titration  of  the  acid  is  undertaken.  The 
U.S.P.  method  is  as  follows:  Pour  about  10  mils  of  aromatic 
sulphuric  acid  into  a  tared  flask,  stopper  and  weigh,  transfer 
to  a  small  flask  using  60  mils  of  distilled  water  to  rinse  the 
weighing-flask,  and,  having  connected  the  flask  with  a  reflux 
condenser,  boil  the  liquid  for  six  hours;  when  cold,  dilute  it 
with  distilled  water  to  about  100  mils,  and  titrate  with  normal 
potassium  hydroxid  V.S. ,  using  methyl  orange  as  indicator. 
Calculate  as  in  preceding  assay. 

Phosphoric  Acid  (H3P04  =  98.064).  In  the  assay  of  phos- 
phoric acid  by  du*ect  neutralization  with  standard  KOH, 
the  acid  is  converted  into,  first,  KH2PO4,  then  K2HPO4, 
and  finally  into  the  normal  K3PO4.  We  have  no  indicator 
which  reliably  shows  the  completion  of  the  neutralization, 
i.e.,  the  formation  of  the  tribasic  K3PO4.  Litmus  cannot 
be  used  as  indicator  here  for  the  dipotassic  or  disodichydric 
phosphate  (K2HPO4  or  Na2HP04)  which  is  formed  is 
slightly  alkaline  towards  litmus;  the  s^me  is  true  of  most  other 
indicators. 

Thompson,  however,  has  demonstrated  that  this  acid  may 


106      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

be  accurately   titrated  by  standard  alkali  when  using  either 
methyl  orange  or  phenolphthalein,  or  both,  successively. 

If  methyl  orange  is  used  the  color  changes  upon  the  com- 
pletion of  the  formation  of  monobasic  phosphate,  KH2PO4,  as 
per  the  following  equation: 

H3P04  +  KOH  =  KH2P04  +  H20. 

If  phenolphthalein  is  used,  the  color  changes  upon  the  com- 
pletion of  the  formation  of  the  dibasic  phosphate,  KH^04, 

H3P04  +  2KOH  =  K2HP04  +  2H20. 

As  an  example:   A  weighed  quantity  of.  the  acid  is  diluted 

with  water  to  measure   20  mils  and  sufficient  pure  sodium 

chlorid  added  to  saturate  the  solution.     Four  drops  of  methyl 

N 
orange  are  then  introduced  and  the  titration  with  —  KOH 

begun  and  continued  until  the  red  color  changes  to  yellow, 
indicating  the  formation  of  the  monobasic  phosphate 

H3PO4  +  KOH  =   KH2PO4  +  H2O. 

N 
98,064  gms.      56.1  gms.  =  1000  mils  —  V.S. 

N 
Each  mil  of  —  KOH  V.S.  =  0.098064  gm.  of  H3PO4. 

The  use  of  sodium  chlorid  in  this  assay  is  to  decrease  the 
ionization  of  the  acid  salts  produced  in  the  reaction. 

Another  portion  of  the  acid  is  treated  in  like  manner, 
adding  sodium  chlorid  and  titrating,  but  using  phenolphthalein 
as  the  indicator.  The  titration  is  continued  until  a  faint 
permanent  pink  color  appears.  It  is  advisable  to  use  heat, 
or  better .  still,  a  standard  alkali  solution  which  is  quite  free 


ANALYSIS  BY  NEUTRALIZATION  107 

from  CO 2-     The  end-reaction  in  this  case  marks  the  formation 
of  the  dibasic  phosphate 

H3PO4  +   2KOH  =  K2HPO4  +  2H2O. 

2)98.064  2)112.2  jg- 

49.032  gms.        56.1  gms.  =  1000  mils  —  V.S. 

N 
Each  mil  of  —  KOH  V.S.  =  0.049032  gm.  of  H3PO4. 

N 
Just  twice  as  much  of  the  —  KOH  V.S.  will  be  taken  in 

this  assay  as  in  the  foregoing. 

The  two  assays  may  be  combined  as  follows:  A  weighed 
quantity  of  the  acid  is  diluted  with  water  saturated  with 
sodium  chlorid  and  titrated  with  the  normal  alkali  V.S., 
using  methyl  orange  as  indicator  until  the  red  color  of  the 
solution  changes  to  yellow.  The  number  of  mils  is  noted 
and  multiplied  by  0.098064  gm.  A  few  drops  of  phenol- 
phthalein  solution  are  now  added  and  the  titration  continued 
until  a  pale-red  color  appears.  The  total  number  of  mils 
of  normal  alkali  used  in  the  double  titration  is  then  multiplied 
by  0.049032  gm. 

The  U.S. P.  method  for  the  assay  of  phosphoric  acid  is 
given  on  page  127. 

Hypophosphorous  Acid  (HPH2O  2  =  66.06) . 

HPH2O2  +  KOH  =  KPH2O2 + H2O. 

N 
Each  mil  of  —  alkali  represents  0.06606  gm.  of  HPH2O2. 

Nitric  Acid  (HNO3  =  63.02) . 

HN03-hKOH  =  KN03+H20. 

N 
Each  mil  of  —  alkali  represents  0.06302  gm.  of  HNO3. 

Hydrobromic   and   hydriodic   acids  may  be   estimated   in 


108      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

the  same  way  as  the  foregoing,  but  it  is  usually  preferred  to 
estimate  them  by  precipitation  analysis.  This  is  also  true  of 
phosphoric  acid.  Sulphurous  acid  is  best  assayed  by  oxida- 
tion with  iodin. 

Boric    Acid   (H3B03  =  62.02).    This  acid  is  estimated  by 

N 
neutralization   with   —  NaOH     in   the   presence    of   a   large 

quantity  of  glycerin.  (Thompson's  Method,  J.  S.  C.  L,  XII, 
432).  The  addition  of  sufficient  glycerin  to  a  boric  acid 
solution,  so  that  no  less  than  30  per  cent  be  present  throughout 
the  titration,  develops  the  acidity  of  boric  acid  with  regard 
to  phenolphthalein  to  a  great  degree,  and  enables  one  to 
titrate  direct  with  standard  soda  solution.  One  gm.  of  boric 
acid  is  dissolved  in  50  mils  of  water;  to  this  is  added  an  equal 
volume  of  glycerin,  then  a  few  drops  of  phenolphthalein,  and 
the  titration  with  normal  sodium  hydroxid  begun  and  con- 
tinued until  a  pink  color  appears. 

N 
Each  mil  of  —  NaOH =0.06202  gm.  of  H3BO3. 

H3BO3  +  NaOH  =  NaH2B03  +  H2O. 

N 
62.02  40  gms.        in  1000  mils  —  V.S. 

N 
0.06202  gm.  =  I  mil   —  V.S. 

Estimation  of  the  Organic  Acids 

As  the  individual  organic  acids  require  different  indicators, 
the  table  on  page  28  should  be  consulted  in  the  selection  of 
an  indicator  for  a  particular  organic  acid.  Phenolphthalein 
is,  however,  the  most  suitable  for  organic  acids  generally. 

Acetic  Acid  (HC2H302  =  60.03).  Mix  3  gms.  of  the  acid 
with  50  mils  of  water,  add  a  few  drops  of  phenolphthalein 
T.S.,  and  titrate  with  normal  potassium  hydroxid  V.S.  until 


ANALYSIS  BY  NEUTRALIZATION  109 

a  permanent  pale  pink  color  is  obtained,  and  apply  the  fol- 
lowing equation : 

HC2H3O2   +  KOH  =   KC2H3O3   +  H2O. 

60  56.1 

N 
Thus  1000  mils  of  —  KOH  V.S.  will  neutralize  60  gms. 

N 
of  acetic  acid;   therefore  each  mil  of  —  KOH  V.S.  represents 

0.060  gm.  of  acetic  acid. 

If  18  mils  are  required  to  neutralize  3  gms.  of  the  acid,  it 
contains  18X0.06=  1.08  gms.  of  absolute  acetic  acid. 

1.08  X 100 

=  36  per  cent. 

Tartaric    Acid   (H2C4H406=  150.05).    Dissolve  2  gms.  of 

tartaric  acid  in  50  mils  of  distilled  water,  add  a  few  drops  of 

phenolphthalein  and  then  pass  into  the  solution  from  a  burette 

N 

—  potassium  hydroxid  V.S.  until  a  faint  pink  tint  is  acquired 

by  the  solution,  and  apply  the  equation 


H2C4H4O6  +  2KOH  =  K2C4H4O6  +  2H2O. 

2)150  Isj 

75  gms.  =  1000  mils  —  V.S. 

Citric  Acid  (HsCgHsOt^  192.06). 

H3C6H5O7   +  3KOH  =  K3C6H5O7  +  4H2O. 

3)192.06  j^ 

64.01  gms.  =  1000  mils  —  V.S. 

N 
Each  mil  of  —  KOH  represents  0.06401  gm.  of  citric  acid. 

Trichloracetic  Acid  (CCl3C00H=  163.39). 

CCI3COOH  +  KOH  =  CCI3COOK + H2O. 

163.39 


110      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

N 

Each  mil  of  —  KOH  V.S.  represents  0.16339  gm.  of  tri- 
chloracetic acid. 

The  other  organic  acids  are  assayed  in  exactly  the  same 
manner  as  that  described  for  the  foregoing  with  the  exception 
of  benzoic  and  salicylic  acids. 

Benzoic  Acid  (HC7H5O2)  and  Salicylic  Acid  (HC7H5O3). 
These  acids  are  assayed  as  follows:  Dissolve  0.5  gm.  of  the 
acid,  previously  dried  to  constant  weight  in  a  desiccator  over 
sulphuric  acid,  in  25  mils  of  diluted  alcohol  which  has  been 
previously  neutralized  with  tenth-normal  potassium  hydroxid 
V.S.,  phenolphthalein  T.S.  being  used  as  indicator.  Titrate 
this  solution  with  tenth-normal  barium  hydroxid  V.S.,*  using 
phenolphthalein  as  indicator. 

The  reactio::>s  are 

2HC7H502  +  Ba(OH)2  =  Ba(C7H502)2  +  2H20. 

2)244.10 
10)122.05  ^ 

1 2.205  =  to  1000  mils  —  Ba(0H)2  V.S. 

N 
Each  mil  of  —   Ba(0H)2  V.S.  represents  0.012205  gm.  of 

benzoic  acid  and  0.013805  gm.  of  salicylic  acid. 

Lactic  Acid  (HC3H503  =  90.05).  A  definite  weight  is 
treated  with  an  excess  of  normal  potassium  hydroxid  V.S. 
and  boiled  for  twenty  minutes.  The  boiling  solution  is  then 
titrated  with  normal  sulphuric  acid  V.S.  The  quantity  of  the 
latter  consumed  deducted  from  the  quantity  (in  mils)  of  normal 
potassium  hydroxid  V.S.  originally  added,  gives  the  quantity 
of  the  alkali  solution  which  represents  the  lactic  acid  being 
assayed. 


*  See  page  loi. 


ANALYSIS  BY  NEUTRALIZATION 


111 


TABLE  SHOWING  QUANTITY  OF  SUBSTANCE  TO  BE  TAKEN  FOR 
ANALYSIS  IN  DIRECT  PERCENTAGE  ESTIMATIONS. 


Molec- 
ular 
Weight. 


Quantity  to  be 
taken  so  that 

each  mil  of  — 
I 
V.S.  will  rep- 
resent I  %. 


Percentage 
strength  of 

Official 
Substance. 


Acid,  acetic,  HC2H3O2 

"     boric,  H3BO3 

' '     citric,  H3C6H5O7+H2O .> 

' '     hydrobromic,  HBr.     Dil 

"     hydrochloric,  HCl 

"     hydriodic,  HI.     Dil . 

' '     hypophosphorous,  HPH2O2 

"     lactic,  HC3H5O3 

"     nitric,  HNO3 

*'     oxalic,  H2C,04+2H20 

'  *     phosphoric,  H3PO4 

* '     phosphoric  with  methyl  orange 

' '     phosphoric  with  phenolphthalein 

' '     sulphuric,  H2SO4 

' '     tartaric,  H2C4H4O6 

* '     trichloracetic,  HC2CI3O2 

Ammonium  carbonate,  N3H11C2O6 

Ammonia  water,  NH3 

Ammonia  water,  stronger,  NH3 

Lime  water,  Ca(0H)2 

Lithium  carbonate,  Li2C03 

citrate,  Li3C6H607+4H20 

Potassium  acetate,  KC2H3O2 

bicarbonate,  KHCO3 

'  *        bitartrate,  KHC4H4O6 

**        carbonate,  K2CO3 

' '        citrate,  K3C6H5O7+H2O 

hydroxid,  KOH 

"        hydroxid,  liquor,  KOH 

"        sodium  tartrate 

KNaC4H406+4H20 

Sodium  acetate,  NaCaH.Oa+sHzO 

benzoate,  NaC7H602 

'  *       bicarbonate,  NaHCOg 

*  *       carbonate,  Na2C03 


60.03 

62.02 

210.08 

80.93 

36.47 
127.93 
66.04 
90.05 
63.01 
126.05 
98.06 


6.03 
6.2 
7.0 
8.093 
3  647 
12.793 
6.604 
9.0 
6.301 
6.3 


gms 


98.09 
150.05 
163.38 
15711 
17.03 
17.03 
74.09 
73.88 
281 .92 
98.12 
100. 1 
188. 1 
138.2 
32436 
.56.1 
56.1 

282.  2 
136.0 
144.0 
84.0 
106.0 


9.806 
4  903 
4  903 

5 

6338 

25 

7 

7 

3  704 
369 

9-397 

9.812 
10.01 
18.81 

6.91 
10.81 

5-61 

5.61 

14.10 

136 

14.4 

8.4 

5-3 


36. 
995 
99- 5 
10. 

32. 
10. 
30. 

85. 
68. 

85. 


Referring  to  the  table  it  will  be  seen  that  if  the  quantities 
indicated  are  taken  for  analysis,  the  amount  of  standard 
solution  required  for  substances  of  high  percentage  strength 
will  be  very  large  (in  some  cases  over  99  mils),  while  for  sub- 


I 


112      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

stances  of  low  percentage  strength,  as  for  instance  lime  water, 
so  small  a  volume  of  standard  solution  is  required  as  to  be 
unreadable  (0.14  mil).  It  is  therefore  advisable  to  take  for 
analysis  a  smaller  quantity  of  high  percentage  substances  and 
a  larger  quantity  of  such  substances  as  contain  a  low  percent- 
age. It  is  usually  best  to  so  adjust  it  that  no  less  than  10 
nor  more  than  30  mils  of  the  standard  solution  be  required. 
For  example:  In  the  case  of  citric  acid,  instead  of  taking 
for  analysis  7  gms.  it  will  be  better  to  take  one-fourth  of  this 
quantity,  then  each  mil  of  the  standard  solution  used  will 
represent  4  per  cent,  and  only  one-fourth  as  much  will  be 
required,  i.e.,  24.9  mils  instead  of  99.5  mils.  Again,  in  the 
case  of  lime  water,  if  37.04  gms.  are  taken  instead  of  3.704 

N  ... 

gms.,   1.4  mils  of  the  —  standard  solution  will  be  required, 

which  is  better  than  0.14  mil,  but  in  this  case  it  will  be  still 
better  to  use  a  decinormal  (  —  I  solution,  then  37.04  gms.  of 

lime  water  would  require  for  neutralization  just  14  mils  of 

N 
the  —  acid  V.S. 
10 

If  half  the  quantity  indicated  in  the  table  is  taken,  then 
each  mil  of  the  standard  solution  would  represent  2  per  cent. 
If  one-tenth  the  quantity  is  taken  each  mil  will  represent  ic 
per  cent.  If  double  the  quantity  is  taken  each  mil  will  represent 
0.5  per  cent,  etc. 


CHAPTER  IX 

ANALYSIS  BY  PRECIPITATION 

The  general  principle  of  this  method  is  that  the  deter- 
mination of  the  quantity  of  a  given  substance  is  effected  by 
the  formation  of  a  precipitate,  upon  the  addition  of  the 
standard  solution  to  the  substance  under  examination.  There 
are  three  ways  of  determining  the  end-reaction  in  precipi- 
tation analyses: 

1,  By  adding  the  standard  solution  until  it  ceases  to  produce 
any  more  precipitate,  as  in  the  estimation  of  silver  by  standard 
sodium  chlorid,  and  the  estimation  of  haloid  salts  and  acids 
by  means  of  standard  silver  nitrate.  The  application  of  this 
ending  is  almost  limited  to  the  above  estimations,  because 
in  these  only  can  accurate  results  be  obtained.  The  silver 
halids  formed  are  not  only  quite  insoluble,  but  they  have  a 
tendency  to  curdle  closely  upon  shaking  (especially  in  acid 
solutions),  and  thus  leave  a  clear  supernatant  liquid  in  which 
any  further  precipitation  can  readily  be  seen.  Most  of  the 
other  precipitates,  such  as  barium  sulphate,  calcium  oxalate, 
etc.,  although  heavy  and  insoluble,  are  so  finely  divided  and 
powdery  that  they  do  not  readily  subside. 

2.  By  the  use  of  an  indicator,  as  in  the  estimation  of  haloid 
salts  by  means  of  standard  silver  nitrate  solution,  using  neutral 
potassium  chromate  as  the  indicator.  The  latter  is  added  to 
the  haloid  solution  (which  must  be  neutral),  and  the  silver 
nitrate  V.S.  delivered  into  the  mixture  until  a  permanent  red 
color  (silver  chromate)  is  produced.  Silver  nitrate  reacts  by 
preference  with  the  halogen,  and  does  not  react  with  the  chro- 

113 


114      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

mate  until  the  halogen  has  been  entirely  precipitated.  Hence 
the  production  of  a  permanent  red  color  in  the  precipitate 
marks  the  completion  of  the  precipitation  of  the  halogen. 

Another  illustration  is  in  the  estimation  of  silver  by  sul- 
phocyanate  solution,  using  ferric  alum  as  indicator.  The 
sulphocyanate  produces  with  the  silver  a  white  precipitate 
of  silver  sulphocyanate,  but  when  the  precipitation  of  silver 
is  complete  the  sulphocyanate  reacts  with  the  ferric  alum 
present  and  a  red  ferric  sulphocyanate  appears  and  marks  the 
end-point.  On  the  other  hand,  the  indicator  may  be  used 
externally,  i.e.,  alongside  of  the  liquid  being  analyzed,  a  drop 
of  the  latter  being  brought  in  contact  with  a  drop  of  the  indi- 
cator at  'frequent  intervals  in  the  course  of  the  titration,  as  in 
the  estimation  of  phosphoric  acid  by  means  of  uranium  nitrate 
solution,  in  which  potassium  ferrocyanide  is  used  as  indicator. 

3.  By  adding  the  standard  solution  until  the  first  appearance 

of  a  precipitate,  as  in  the  estimation  of  cyanogen  by  silver 

nitrate  solution,  and  the  estimation  of  chlorin  by  mercuric 

nitrate  V.S.      In  these  estimations   the  standard  solution  is 

added  to  the  solution  of  the  substance  under  analysis  until  a 

precipitate  appears. 

/N\ 
Preparation    of    Decinormal  ( — )  Silver   Nitrate  (AgNO.s 

N    • 
=  169.89;  —  V.S.  =  16.989  gms.  in  1000  mils).     Dissolve  16.989 

gms.  of  pure  silver  nitrate*  in  sufficient  water  to  make,  at 
or  near  25°  C.  (77°  F.),  exactly  1000  mils.  One  liter  of  this 
solution  thus  contains  -^  of  the  molecular  weight  in  grams  of 
silver  nitrate.     It  is  therefore  a  decinormal  solution. 

If  pure  crystals  of  silver  nitrate  are  not  readily  obtainable, 
and  pure  sodium  chlorid  is  at  hand,  a  solution  of  the  silver 

*  This  should  be  pulverized  and  dried  at  1 20°  C.  for  half  an  hour  in  a 
covered  crucible  before  weighing. 


< 


ANALYSIS  BY  PRECIPITATION  115 


I  titrate  may  be  made  of  approximate  strength,  a  little  stronger 
ban  necessary,  and  then  standardized  by  means  of  the  sodium 
hlorid,  as  follows:  0.11692  gm.  of  sodium  chlorid  is  dissolved 
in  distilled  water,  and  a  burette  filled  with  the  solution  of 
silver  nitrate  to  be  standardized.  The  silver  solution  is  now 
slowly  added  from  the  burette  to  the  sodium  chlorid  solution 
contained  in  a  beaker  until  no  more  precipitate  of  silver  chlorid 
is  produced. 

If  neutral  potassium  chromate  is  used  as  an  indicator, 
the  end  of  the  reaction  is  shown  by  the  appearance  of  yellowish- 
red  silver  chromate.  This  indication  is  extremely  delicate. 
The  silver  nitrate  does  not  act  upon  the  chromate  until  all 
of  the  chlorid  is  converted  into  silver  chlorid. 

In  the  above  reaction  20  mils  of  silver  nitrate  should  be 
required.  But  since  the  silver  nitrate  solution  is  too  strong, 
less  of  it  will  complete  the  reaction,  and  the  1  solution  must 
be  diluted  so  that  exactly  20  mils  will  be  required  to  precipitate 
the  chlorin  in  0.11692  gm.  of  NaCl. 

Thus  if  17  mils  are  used,  each  17  mils  must  be  diluted 
to  20  mils,  or  each  170  mils  to  200  mils,  or  the  entire  remain- 
ing solution  in  the  same  proportion. 

After  dilution  a  fresh  trial  should  always  be  made. 

Silver  nitrate  solution  should  be  kept  in  dark  amber- 
colored,  glass-stoppered  bottles,  carefully  protected  from 
dust. 

The  U.S.P.  IX  directs  the  use  of  pure  metallic  silver  (foil, 
wire,  or  powder)  for  preparing  this  solution.  Into  a  flask  con- 
taining 10.788  gms.  of  pure  silver  introduce  gradually  about 
30  mils  of  nitric  acid  (sp.gr.  about  1.403  at  25°  C.)  or  sufficient 
for  complete  solution.  Thoroughly  rinse  the  neck  of  the  flask 
and  the  funnel  which  is  used..  Evaporate  the  solution  to 
dryness,  carefully  protecting  it  from  dust.  Then,  after  drying 
in  an  air  oven  at  about  120°  C,  for  ten  minutes,  dissolve  the 


116      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

residue  in  sufficient  distilled  water  to  measure  exactly  looo 
mils  at  25°  C. 

N  N 

Decinormal  —  Sodium  Chlorid   (NaCl  =  58.46;    —  V.S. 

=  5.846  gms.  in  1000  mils).  Dissolve  5.846  gms.  of  pure 
sodium  chlorid  in  enough  water  to  make  exactly  1000  mils 
at  the  standard  temperature. 

Check  this  solution .  with  decinormal  silver  nitrate.  The 
two  solutions  should  correspond,  volume  for  volume. 

Pure  Sodium  Chlorid  may  be  prepared  by  passing  into 
a  saturated  aqueous  solution  of  the  purest  commercial  sodium 
chlorid  a  current  of  dry  hydrochloric  acid  gas.  The  crystal- 
line precipitate  is  then  separated  and  dried  at  a  temperature 
sufficiently  high  to  expel  all  traces  of  free  acid. 

N 
The  method  of  standardizing  —  NaCl  solution  is  as  follows : 

10 

0-33978  gm.  of  silver  nitrate  is  dissolved  in  10  mils  of  dis- 

N 
tilled  water,  and  the  solution  carefully  titrated  with  —  NaCl 

•    -^  10 

V.S.  until  precipitation  ceases.    Twenty  mils  of  the  standard 

solution  should  be  required. 

AgNOs  +  NaCl  =  AgCl  +  NaNOs. 

10)169.89     10)58.46  j^ 

16.989  gms.  5.846  gms.,  or  1000  mils  —  NaCl  V.S. 

Each  mil  of  the  standard  solution  represents  0.016989  gm. 
of  pure  AgNOa. 

0.016989X20=0.33978  gm. 
0.33978X100 


0-33978 


100  per  cent. 


This  solution  may  also  be  standardized  by  residual  tira- 
tion  with  Volhard's  solution. 


ANALYSIS  BY  PRECIPITATION  117 

N 
Decinormal  —  Potassium  Sulphocyanate  (Volhard's  Solu- 

N 
tion)    (KSCN  =  97.18;   —   V.S.  =  9.718    gms.    in    1000   mils). 

Dissolve  10  gms.  of  pure  crystallized  potassium  sulphocyanate 

(thiocyanate)  in  1000  mils  of  water. 

This  solution,  which  is  too  concentrated,  must  be  adjusted 

so  as  to  correspond  exactly  in  strength  with  decinormal  silver 

nitrate  V.S.    For  this  purpose  introduce  into  a  flask  2c  mils 

N 
of  —  AgNOs  V.S.,  3  mils  of  ammonioferric  sulphate  solution, 

and  5  mils  of  diluted  nitric  acid  (10  per  cent  and  free  from 
nitrous  compounds). 

Dilute  the  liquid  with  75  mils  of  distilled  water,  and  titrate 
it  with  the  sulphocyanate  solution. 

At  first  a  white  precipitate  of  silver  sulphocyanate  is 
produced,  giving  the  fluid  a  milky  appearance,  and  then  as 
each  drop  of  sulphocyanate  falls  in  it  is  surrounded  by  a  deep 
brownish-red  cloud  of  ferric  sulphocyanate,  which  quickly 
disappears  on  shaking,  as  long  as  any  of  the  silver  nitrate 
remains  unchanged. 

When  the  point  of  saturation  is  reached  and  the  silvei 
has  all  been  precipitated,  a.  single  drop  of  the  sulphocyanate 
solution  produces  a  faint  brownish-red  color,  which  does  not 
disappear  on  shaking. 

Note  the  number  of  mils  of  the  sulphocyanate  solution 
used,  and  dilute  the  whole  of  the  remaining  solution  so  that 
equal  volumes  of  this  and  of  the  decinormal  silver  nitrate 
will  be  required  to  produce  the  permanent  brownish-red  tint. 
(The  same  tint  of  brown  or  red  to  which  the  volumetric  solution 
is  adjusted  must  be  attained  when  the  solufion  is  used  in 
volumetric  testing.) 

Assuming  that  19  mils  of  the  sulphocyanate  solution  were 


118      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

required  to  produce  the  reaction,  then  each  19  mils  must  be 

diluted  to  make  20  mils,  or  the  whole  of  the  remaining  solution 

in  the  same  proportion. 

Always  make  a  new  trial  after .  the  dilution  to  see  if  the 

N 
solutions  correspond,  e.g.,  50  mils  of  —  silver  nitrate  are  taken, 

and  5  mils  of  ammonioferric  sulphate,  5  mils  of  pure  nitric 
acid  and  200  mils  of  water  are  added,  and  there  should  be 
required  exactly  50  mils  of  the  potassium  sulphocyanate  solu- 
tion. The  same  depth  of  reddish-brown  tint  should  be  ob- 
tained in  all  assays  by  this  method,  as  is  obtained  in  standard- 
izing the  solution. 

Estimation  of  Soluble  Haloid  Salts 

The  estimation  of  these  salts  is  based  upon  the  powerful 
affinity  existing  between  the  halogens  and  silver,  and  the 
ready  precipitation  of  the  resulting  chlorid,  bromid  and  iodid. 
Standard  solution  of  silver  nitrate  is  used  for  this  purpose, 
and  for  the  sake  of  exactness  and  convenience,  is  made  of 
decinormal  strength.  In  some  cases  it  is  advisable  to  use 
centinormal  solutions. 

Mohr*s  Method  with  Chromate  Indicator.  This  method 
is  the  best  to  use,  if  the  haloid  salts  are  in  neutral  solution, 
and  salts  of  lead,  bismuth,  barium  or  iron  are  absent.  If 
the  solution  is  acid  the  indicator  is  inadmissable,  in  that  acids 
have  a  solvent  action  upon  silver  chromate  and  thus  prevent 
the  end-reaction  from  being  clearly  and  accurately  observed. 
If  the  above-mentioned  metals  are  present,  the  indicator  is 
likewise  useless,  as  these  bases  form  insoluble,  highly  colored 
compounds  with  the  chromate.  The  neutral  potassium  chro- 
mate (yellow  chromate)  which  is  used  as  the  indicator  must  be 


ANALYSIS  BY  PRECIPITATION  119 

*ee  from  chlorid  *  and  should  be  used  in  the  form  of  a  lo 
)er  cent  solution. 

In  the  volumetric  analysis  of  soluble  haloid  salts  (chlorids, 
)romids  and  iodids)  0.5  gm.  of  the  well-dried  salt  is  dissolved 

i  40  mils  of  water  in  a  beaker.  This  is  placed  upon  a  white 
jurface  and  a  few  drops  of  the  chromate  indicator  (or  suffi- 
^ent  to  give  the  solution  a  pale  yellow  tint),  added.     The 

"    N  . 

decinormal  —  silver  nitrate  solution  is  then  added  cautiously 
10  ^ 

from   a   burette,   stirring   constantly   until   a   permanent   red 
tint  is  produced.     The  red  tint  is  due  to  the  formation  of 
silver  chromate,  which  does  not  appear  permanent  until  the 
last  trace  of  halogen  has  been  precipitated. 
The  reactions  are  as  follows: 

NaCl  +  AgNOa  =  AgCl  +  NaNOs 
and 

K2Cr04  +  2  AgNOa  =  Ag2Cr04  +  2KNO3. 

If  the  solution  to  be  estimated  is  acid  it  should  be  accu- 
rately neutralized  with  ammonia,  or  sodium  or  calcium  car- 
bonate. If  it  is  alkaline  in  reaction  it  should  likewise  be 
neutralized,  using  acetic  acid  for  this  purpose. 

In  the  estimation  of  bromids  and  iodids  *t  must  not  be 
forgotten  to  take  into  account  the  invariable  presence  of 
chlorids  as  an  impurity. 

The  method  in  detail  is  exemplified  in  the  following  assay : 

Estimation  of  Sodium  Chlorid.  One  gm.  of  the  well-dried 
sodium  chlorid  is  dissolved  in  sufficient  distilled  water  to 
measure  100  mils.     Of  this  solution  10  mils  (representing  o.i 

*  The  presence  of  chlorid  in  the  chromate  solution  may  be  determined 
by  adding  a  small  quantity  of  silver  nitrate  solution,  and  then  some  nitric 
acid.     If  the  red  precipitate  dissolves  completely  and  leaves  ^  clear  solutioiceCl 
chlorid  is  absent.     If  it  is  found  to  be  present  it  may  be  removed  '  TTronlrl 
addition  of  a  few  drops  of  silver  nitrate  solution,  and  filtering,  witV  ' 

any  nitric  acid.  ^^^  ^^^' 


120      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

gm.  of 'the  salt)  is  taken,  a  few  drops  of  neutral  potassium 

N 
chromate   solution   added,    and   then   the   —   silver  solution 

lO 

delivered  from  a  burette  with  constant  stirring  or  shaking 
until  the  chlorid  is  entirely  precipitated,  as  evidenced  by  the 
formation  of  a  permanent  red  color  (silver  chromate).  The 
equation  is 

,       NaCl  +  AgNOs  =  AgCl  +  NaNOs. 

*■"■"      10)58.46   10)160-89  j^ 

5.846  16.989  gms.  =  1000  mils  —  V.S. 

N 
Thus  each  mil  of  —  V.S.  represents  0.005846  gm.  of  NaCl. 

If  in  the  above  assay  17  mils  of  the  silver  solution  were  re- 
quired, then  17  X0.005846  gm.  =0.099382  gm.  or  99.382  per  cent. 

0.099382  X 100 

' =  99-3o2  per  cent. 

Titration  without  an  Indicator — Gay-Lussac's  Method.  In 
this  method  no  indicator  is  used,  the  standard  solution 
being  added  until  it  ceases  to  produce  any  further  precipi- 
tation. This  method  is  applicable  to  acid  solution  of  the 
haloid  salts,  c^nd  to  the  haloid  acids— hydrochloric,  hydro- 
bromic  and  hydriodic;  also  to  the  estimation  of  silver  by 
standard  solution  of  sodium  chlorid.  The  method  is  carried 
out  in  hot  solutions,  slightly  acidulated  with  nitric  acid,  in 
order  to  facilitate  the  precipitation  of  the  silver  halid.  The 
haloid  acids  are  neutralized  with  an  alkali  and  then  slightly 
acidulated  with  nitric  acid  before  the  titration  is  begun.  The 
calculations  are  precisely  like  those  in  the  foregoing  assays. 
Volhard's  or  Sulphocyanate  Method.  This  method  depends 
on  completely  precipitating  the  halogen  in  the  presence  of 
^^  ^  ^^  cid,  by%  measured  excess  of  standard  silver  nitrate 
Aj  and  then  estimating  the  excess  of  silver  by  retitrating 


ANALYSIS  BY  PRECIPITATION  121 

with  standard  sulphocyanate  solution,  using  ferric  alum  as 
an  indicator. 

The  sulphocyanate  has  a  greater  affinity  for  silver  than 
it  has  for  iron,  and  therefore,  so  long  as  any  silver  is  in  solu- 
tion, the  sulphocyanate  will  combine  with  it  and  form  a  pre- 
cipitate of  silver  sulphocyanate. 

As  soon  as  the  silver  is  all  taken  up,  the  sulphocyanate 
will  combine  with  the  ferric  alum  and  strike  a  brownish-red 
color. 

The  sulphocyanate  solution  is  to  be  made  of  such  strength 
that  it  correspor  is  with  the  silver  solution,  volume  for 
volume. 

The  difference  between  the  volume  of  silver  solution  origi- 
nally added  and  the  volume  of  sulphocyanate  solution  used, 
will  give  the  volume  of  silver  solution  equivalent  to  the  haloid 
salt  present. 

This  method  has  the  advantage  over  the  direct  method 
for  haloids  with  chromate  indicator,  in  that  it  may  be  used 
in  the  presen':e  of  nitric  acid.  It  thus  enables  one  to  estimate 
the  haloids  in  the  presence  of  phosphates  or  other  salts  which 
precipitate  silver  in  neutral  but  not  in  acid  solutions,  and  also 
in  that  the  presence  of  barium,  bismuth,  lead,  iron  and  other 
metals  do  not  interfere,  as  they  do  with  the  chromate  in 
Mohr's  method.  The  presence  of  mercury,  however,  exerts 
a  disturbing  influence  upon  the  end-reaction.  The  nitric  acid 
acidulates  the  solution  and  thus  facilitates  the  precipitation 
of  silver  by  the  halogens,  and  prevents  its  precipitation  by 
other  substances.  The  quantity  of  nitric  acid  employed  is  of 
no  great  importance,  except  in  the  case  of  iodids  (because 
silver  iodid  is  sHghtly  soluble  in  nitric  ajid).  Usually  suffi- 
cient of  the  acid  is  added  to  just  remove  the  color  produced 
by  the  indicator.  A  very  large  excess  of  the  acid  would, 
however,  interfere  with  the  proper  determination  of  the  end- 


122       ANALYSIS  BY  OXIDATION  AND  REDUCTION 

reaction,  in  that  it  to  a  slight  extent  prevents  the  formation 
of  ferric  suli)hocyanate.  In  the  estimation  of  iodids  by  this 
method,  the  nitric  acid  should  be  added  after  the  standard 
silver  solution,  while  in  the  case  of  the  other  haloid  salts  the 
acid  may  be  added  before. 

The  indicator  also  should  be  added  after  the  standard 
silver  solution,  when  estimating  iodids,  because  being  a  ferric 
salt  it  is,  like  nitric  acid,  capable  of  liberating  iodin. 

The  solutions  required  for  this  method  are: 
(I)  Decinormal  Silver  Nitrate  (page  114); 
(II)  Decinormal  Potassium  Sulphocyanate  (page  117); 

(III) '  Ferric  Aliun  Solution.     (The  indicator.) 

This  is  a  10  per  cent  aqueous  solution  of  ferric-ammonium 
sulphate,  FeNH4(S04)2  +  i2H20. 

(IV)  Nitric  Acid  (C.P.).  This  must  be  free  from  nitrous 
acid.  If  it  or  any  of  the  lower  oxids  of  nitrogen  are  present 
they  may  be  removed  by  diluting  with  one-fourth  part  of 
water  and  boiling  until  colorless. 

The  process  is  exemplified  in  the  following  assays: 

Estimation  of  Potassium  Bromid.  Dissolve  about  0.4  gm. 
of  the  salt  accurately  weighed  in  25  mils  of  distilled  water  in 
a  glass-stoppered  flask.  Add  50  mils  of  tenth-normal  silver 
nitrate  V.S.,  then  add  2  mils  of  ammonio-ferric  sulphate  T.S. 
and  2  mils  of  nirtic  acid,  shake  well  and  finally  determine  the 
excess  of  the  silver  nitrate  V.S.  by  titrating  with  tenth -normal 
potassium  sulphocyanate  V.S.  until  a  permanent  reddish  tint 
pervades  the  supernatant  liquid. 

The  difference  between  the  number  of  mils  of  tenth-normal 
silver  nitrate  V.S.  added  and  the  number  of  mils  of  tenth- 
normal potassium  sulphocyanate  used,  multiplied  by  0.011902 
gm.,  gives  the  weight  of  pure  KBr  in  the  quantity  of  salt  taken 
for  analysis. 


ANALYSIS  BY  PRECIPITATION  123 

Assuming  that  in  the  foregoing  analysis  16.9  mils  of  the 
sulphocyanate  solution  were  used,  then  50-16.9  =  33.1  mils 
of  the  silver  nitrate  solution  reacted  with  the  potassium  bromid. 
Hence  0.011902X33.1=0.3939  gm.  of  KBr,  or  98.5  per  cent. 
The  reactions  are 

KBr + AgNOa  =  AgBr  +  KNO3. 

10000)119.02  ^  '-fc^ 

.011902  gm.  =  i  mil  of  —  AgNOa  V.S. 

lodids  are  estimated  in  exactly  the  same  manner. 

When  this  method  is  used  for  the  estimation  of  chlorids, 
however,  the  precipitated  silver  chlorid  must  be  removed  by 
filtration,  because  of  the  action  of  ferric  sulphocyanate  upon 
silver  chlorid,  which  causes  the  results  of  the  analysis  to  be 
too  high. 

In  the  case  of  silver  bromid  or  iodid  no  such  reaction  takes 
place,  or  if  it  does,  the  reaction  is  so  slow  as  not  to  interfere 
in  the  least  with  the  getting  of  accurate  results.  Therefore, 
when  this  method  is  used  for  the  determination  of  bromids 
or  iodids,  there  is  no  need  for  filtering  to  remove  the  pre- 
cipitate. 

The  procedure  in  detail  is  exemplified  by  the  following: 

Estimation  of  Sodium  Chlorid.  Dissolve  about  0.2  gm. 
of  the  salt  accurately  weighed  (or  a  near  equivalent  of  its 
solution)  in  25  mils  of  distilled  water  in  a  200-mil  graduated 
flask.  Add  50  mils  of  tenth-normal  silver  nitrate  V.S.  to  the 
solution,  and  after  adding  5  mils  of  nitric  acid,  add  sufficient 
distilled  water  to  make  200  mils  and  mix  well.  Then  filter 
through  a  dry  filter  into  a  dry  flask,  and  reject  the  first  20 
mils  of  the  filtrate.  Collect  the  next  100  mils  of  filtrate,  add 
2  mils  of  ammonio-ferric  sulphate  T.S.  and  titrate  it  with 
tenth-normal  potassium  sulphocyanate  until  a  permanent  red- 
dish supernatant  liquid  is  obtained.     Multiply  the  number  of 


124      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

mils  of  the  potassium  sulphocyanate  solution  required  by  2 
and  subtract  this  figure  from  50  (the  number  of  mils  of  tenth- 
normal silver  nitrate  V.S.  added).  The  difference  multiplied 
by  the  decinormal  factor  of  sodium  chlorid,  0.C05046  gm,, 
gives  the  weight  of  pure  NaCl  in  the  amount  of  salt  taken  for 
analysis. 


Estimation  of  Haloid  Acids 

These    acids,    namely,     hydrochloric,     hydrobromic    and 

hydriodic,  may  be  estimated  by  Gay-Lussac*s  method  above 

described,   or   they   may   be    estimated   by   Mohr's    Method. 

using  neutral  potassium  chromate  as  an   indicator.     In   this 

case   it   is   necessary   to   carefully   neutralize   the   acid    with 

N 
ammonia   and   then   titrate   with    —   silver   nitrate   solution, 

10  ' 

using  a  few  drops  of  chromate  as  indicator,  in  the  manner 
described  in  the  foregoing  assays.  They  may  also  be  esti- 
mated by  Volhard's  Method,  in  which  an  excess  of  the  standard 
silver  nitrate  solution  is  used,  in  the  presence  of  nitric  acid, 
and  the  amount  of  the  excess  determined  by  residual  titration 
with  potassium  sulphocyanate,  using  ferric  alum  as  the  indi- 
cator. This  method  is  especially  useful  for  iodids  and  hydri- 
odic acid,  in  that  the  nitric  acid  need  not  be  added  until  after 
an  excess  of  silver  nitrate  solution  is  used,  and  thus  liberation 
of  iodin  by  the  nitric  acid  avoided. 

The  estimation  of  the  haloid  acids  may  also  be  effected 
by  neutralization  with  standard  alkali,  in  the  same  way  as 
other  acids,  but  since  hydrobromic  and  hydriodic  acids  are 
now  frequently  prepared  by  the  method  of  Fothergill,  in 
which  potassium  bromid  or  potassium  iodid  (according  to 
the  acid  to  be  made)  is  brought  in  contact  with  tartaric  acid 
(as  shown  in  the  equation),  an  excess  of  the  latter  acid  is 


ANALYSIS  BY  PRECIPITATION  125 

unavoidably  present,  and  hence  the  neutraHzation  method  is 
inapplicable. 

KI  +  H2C4H4O6  =  KHC4H4O6  +  HI 

Potassium        Tartaric  acid  Potassium  Hydriodic 

lodid  Bitartrate  Acid 

KBr  "  *^  HBr 

Assay  of  Hydrobromic  Acid,  Using  Chromate  as  Indicator. 

Ten  gms.  of  hydrobromic  acid  are  diluted  with  sufficient  dis- 
tilled water  to  make  ico  mils.  Ten  mils  of  this  solution, 
representing  i  gm.  of  the  acid,  is  exactly  neutralized  with 
diluted  ammonia  water  (using  litmus  solution  as  indicator); 
3   drops  of  neutral  potassium  chromate  solution  are  added, 

N 
and  then  the  —  silver  nitrate  run  in  from  a  burette  until  the 
10 

solution  acquires  a  permanent  red  tint.  The  following  equa- 
tion is  then  applied: 

HBr  +  AgNOs  =  AgBr  +  HNO3. 

10)80.92  j^ 

8.092  gms.  =  1000  mils  —  V.S. 

If  the  assay  is  to  be  made  by  the  direct  percentage  method, 
8.09  mils  of  the  solution  (10  gms.  in  100  mils)   (representing 
0.809  g^s.  of  the  acid)  should  be  taken,  in  which  case  each  • 
mil  of  the  standard  silver  solution  consumed  will  at  once  indi- 
cate I  per  cent. 

Hydriodic  and  hydrochloric  acids  may  also  be  assayed 
by  this  method. 

Assay  of  Hydriodic  Acid  by  the  Sulphocyanate  Method. 
Introduce  into  a  200-mil  stoppered  flask  2  gms.  of  the  acid, 
add  50  mils  of  distilled  water  and  25  mils  (accurately  measured) 
of  decinormal  silver  nitrate,  shake  thoroughly,  and  then  add 
3  mils  of  nitric  acid,  and  heat  on  a  water-bath  until  the  pre- 


126      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

cipitate  has  a  bright  yellow  color.  Then  cool  and  add  2  mils 
of  the  ammonio-ferric  sulphate  T.S.,  and  finally  the  decinormal 
potassium  sulphocyanate  run  in  slowly  from  a  burette,  until 
a  permanent  reddish-brown  tint  is  produced.  Note  the  num- 
ber of  mils  of  sulphocyanate  solution  employed. 

Deduct  this  from  the  25  mils  of  silver  solution  added,  and 
multiply  the  remainder  by  the  factor  for  HI,  which  is  0.012793. 

I.  HI     +     AgNOs     =     Agl     +     HNO3. 

10)127.93  io)i6q.8q  j^ 

12.793  gm3.     16.989  gms.  =  1000  mils  —  V.S. 

N 
0.012793  gm.  of  HI  =  I  mil  —  V.S. 
10 

li.  AgN03  +  KSCN  =  AgSCN+KN03. 

IIL    2FeNH4(S04)2+6KSCN 

=  2Fe(SCN)3  +  (NH4)2S04 +3K2SO4. 

The  reddish-brown  color  which  marks  the  end-reaction 
is  due  to  the  formation  of  Fe(SCN)  3  ferric  sulphocyanate. 

Assuming  that  in  the  above  titration  10.2  mils  of  decinormal 
sulphocyanate  were  employed,  then  25  mils  - 10.2  =  14.8  mils. 

0.912793X14.8  =  0.1893  gm. 

0.1893 X 100 

=  940  per  cent. 

Assay  of  Syrup  of  Hydriodic  Acid.  Twenty-five  mils  of 
the  syrup  are  introduced  into  a  stoppered  flask,  100  mils  of 
distilled  water  are  added,  followed  by  40  mils  of  decinormal 
silver  nitrate  and  the  mixture  thoroughly  shaken.  Five  mils 
of  diluted  nitric  acid  and  3  mils  of  the  ferric  alum  solution 
are  now  added,  and  after  again  shaking  the  mixture  it  is 
titrated  with  decinormal  potassium  sulphocyanate  until  a  per- 
manent reddish-brown  tint  appears. 


ANALYSIS  BY  PRECIPITATIONj  127 

OTHER   ASSAYS   BY   VOLHARD'S   METHOD 

Assay  of  Syrup  of  Ferrous  lodid  U.S.P.  Dilute  about 
lo  gms.  of  the  syrup,  accurately  weighed,  with  30  mils  of  dis- 
tilled water,  add  50  mils  of  tenth-normal  silver  nitrate  V.S. 
and  5  mils  of  nitric  acid  and  heat  on  a  water-bath  until  the 
precipitate  of  silver  iodid  is  yellow.  Cool,  add  2  mils  of  ferric 
ammonium  sulphate  T.S.  and  determine  the  residual  silver 
nitrate  by  titration  with  tenth-normal  potassium  sulphocyanate 
V.S.  A  permanent  reddish-brown  tint  marks  the  end  point. 
Not  less  than  16  mils  nor  more  than  19.3  mils  of  the  latter 
should  be  required. 

The  quantity  of  tenth-normal  sulphocyanate  so  used  de- 
ducted from  50  mils  (the  quantity  of  tenth-normal  silver  nitrate 
V.S.  taken  in  the  assay)  gives  the  quantity  of  the  latter  (in  mils) 
which  reacted  with  the  ferrous  iodid. 

Each  mil  of  tenth-normal  silver  nitrate  V.S.  corresponds  to 
0.015484  gm.  of  Fel2. 

Fel2    +  2AgN03   =  2AgI  -f  Fe(N03)2. 

2)_30Q.68_ 

10)154.84  J^ 

15.484  gms.  =  1000  mils  —  AgNOs  V.S. 

Assuming  that  in  the  foregoing  assay  16  mils  of  the  tenth- 
normal sulphocyanate  were  used,  then  50  mils  - 16  mils  =  34  mils. 

0.015484X34  =  0.520456  gm. 
0.520456X100 


10 


=  5.20  per  cent. 


Assay  of  Phosphoric  Acid  U.S.P.  The  neutralization 
method  for  the  assay  of  phosphoric  acid  has  been  discarded  in 
the  U.S.P.  IX,  and  the  precipitation  with  standard  silver  nitrate 
is  recommended.     An  excess  of  the  latter  solution  is  added  to 


128      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

the  neutralized  phosphoric  acid,  and  the  residual  silver  solution 
determined  by  titration  with  standard  sulphocyanate. 

The  procedure  is  as  follows :  An  accurately  weighed  quantity 
of  the  acid  (about  o.i  gm.)  is  diluted  with  lo  mils  of  distilled 
water,  in  a  loo-mil  flask.  One  drop  of  phenolphthalein  T.S. 
is  added,  and  the  solution  carefully  neutralized  with  special 
potassium  hydroxid  solution,  i.e.,  one  free  from  chlorid. 

Fifty  mils  of  tenth-normal  silver  nitrate  V.S.  are  now  added, 
the  mixture  agitated  and  zinc  oxid  (free  from  chlorid)  gradually 
added  in  small  portions  at  a  time,  until  the  liquid  is  neutral 
to  litmus  paper.  It  is  then  diluted  with  sufficient  distilled 
water  to  measure  loo  mils,  again  thoroughly  agitated  and 
filtered  through  a  dry  filter,  collecting  50  mils  of  filtrate.  To 
this  50  mils  of  filtrate  is  added  2  mils  of  nitric  acid  and  2  mils 
of  ammonio-ferric  sulphate  T.S.  and  then  titrated  with  tenth- 
normal potassium  sulphocyanate  V.S.  to  the  production  of 
a  permanent  red  color. 

The  number  of  mils  of  the  latter  multiplied  by  2  is  sub- 
tracted from  50  mils  (the  quantity  of  tenth-normal  silver 
nitrate  V.S.  added)  and  the  difference  is  the  quantity  of  the 
latter  which  reacted  with  the  phosphoric  acid. 

Each  mil  of  tenth-normal  silver  nitrate  V.S.  represents 
0.0032687  gm.  of  H3PO4. 

Notes.  The  neutralization  of  phosphoric  acid  by  potas- 
sium hydroxid  in  the  presence  of  phenolphthalein  is  repre- 
sented by  the  equation 

H3PO4  +  2KOH  =  K2HPO4  +  2H2O. 

An  excess  of  KOH  must  be  avoided,  as  this  reacts  with  silver 
nitrate. 

The  reaction  between  silver  nitrate  and  the  dipotassic 
phosphate  is  usually  represented  by  the  following  equation, 


ANALYSIS  BY  PRECIPITATION  129 

which,  for  the  purposes  of  this  assay,  may  be  considered  suf- 
ficiently accurate  : 

K2HP04+3AgN03=Ag3P04  +  2KN03+HN03. 

Normal  silver  phosphate  being  precipitated. 

It  is  very  probable  that  the  supernatant  liquid  in  this  reac- 
tion contains  also  some  free  phosphoric  acid,  but  that  seems 
to  be  immaterial. 

3K2HPO4  +  6AgN03  =  2Ag3P04  +  6KNO3 + H3PO4. 

The  zinc  oxid  is  added  in  this  assay  for  the  purpose  of 
neutralizing  the  acid  or  acids  liberated  in  the  interaction. 
This  is  necessary  because  silver  phosphate  (Ag3P04)  is  soluble 
in  dilute  nitric  and  phosphoric  acids.  Too  large  an  excess 
of  zinc  oxid  must  be  avoided,  as  it  acts  upon  silver  nitrate, 
precipitating  Ag20  if  left  in  contact  too  long.  It  is  advisable 
for  this  reason  to  filter  as  soon  as  possible  after  the  addition 
of  zinc  oxid. 

If,  as  is  claimed,*  that  phosphoric  acid  is  liberated  by  the 
interaction  of  dipotassic  phosphate  and  silver  nitrate,  the 
zinc  oxid  will  form  with  it,  zinc  phosphate  which  in  a  neutral 
solution  is  transposed  by  the  silver  nitrate  into  silver  phosphate, 
and  zinc  nitrate,  thus  converting  all  of  the  phosphoric  acid 
into  Ag3P04. 

Zinc  oxid  appears  to  be  the  most  suitable  substance  avail- 
able for  this  neutralization,  because  its  action  upon  silver  nitrate 
is  very  slow  and  insignificant,  if  not  in  large  excess. 

Assay  of  Sodium  Phosphate.  This  may  be  performed  by 
the  same  method. 

Into  a  loo-mil  graduated  flask,  introduce  an  accurately- 
weighed  quantity  of  the  salt  (about  0.4  gm.),  dissolve  it  in 

*  J.  Rasin,  J.  A.  C.  S.,  Vol.  33,  page  1103. 


130      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

lo  mils  of  distilled  water,  add  50  mils  of  tenth-normal  silver 
nitrate  V.S.  and  shake  well.  Then  add  gradually  pure  zinc 
oxid  in  small  portions,  until  the  liquid  is  neutral  to  litmus. 
Dilute  the  mixture  with  distilled  water  to  make  100  mils. 
Shake  thoroughly  and  filter  through  a  dry  filter.  Reject  the 
first  20  mils  of  filtrate.  Collect  50  mils,  add  to  it  2  mils  of 
nitric  acid  and  2  mils  of  ammonio-ferric  sulphate  T.S.  and 
titrate  with  tenth-normal  potassium  sulphocyanate  V.S.  to 
the  production  of  a  permanent  red  color. 

Each  mil  of  tenth-normal  silver  nitrate  V.S.  used  corre- 
sponds to  0.C04735  gm.  of  Na2HP04.  The  calculations  are 
as  given  in  preceding  assay. 

Assay  of  Hjrpophosphites.  This  is  performed  by  the  method 
just  described  for  phosphoric  acid  and  sodium  phosphate. 
The  hypophosphites  being  first  oxidized  to  phosphates.  Dis- 
solve about  I  gm.  (accurately  weighed)  of  the  salt  in  10  mils 
of  distilled  water  and  10  mils  of  nitric  acid  and  evaporate 
the  solution  to  dryness,  on  a  water-bath.  Then  add  5  mils 
of  nitric  acid  and  again  evaporate  it  to  dryness  on  a  water- 
bath.  Dissolve  the  residue  in  20  mils  of  distilled  water,  add  a 
drop  of  phenolphthalein  T.S.,  and  sufficient  special  potassium 
hydroxid  .T.S.  (free  from  chlorids)  to  produce  a  pale  pink 
color,  and  then  dilute  the  mixture  to  exactly  100  mils,  with 
distilled  water.  Transfer  10  mils  of  this  solution  to  a  100- 
mil  flask,  add  50  mils  of  tenth-normal  silver  nitrate  V.S.  and 
proceed  from  this  point  as  directed  under  assay  of  sodium 
phosphate. 

Each  mil  of  tenth-normal  silver  nitrate  V.S.  corresponds  to 

0.002836  gm.  of  Ca(PH202)2; 
0.003472  gm.  of  KPH2O2; 
0.0035357  gm.  of  NaPHsOs. 


ANALYSIS  BY  PRECIPITATION  131 

Estimation  of  Cyanogen 

Titration  with  Standard  Silver  Solution  to  First  Appearance 
of  a  Precipitate— Liebig's  Method.  This  gives  fairly  accurate 
results.  The  cyanogen  must  be  in  the  form  of  an  alkali  salt 
and  in  an  alkaline  solution.  If  hydrocyanic  acid  is  to  be 
estimated,  it  must  be  made  alkaline  by  the  addition  of  potas- 
sium or  sodium  hydroxid.  The  standard  silver  solution  is 
then  added  cautiously  and  with  constant  stirring  until  a  per- 
manent precipitate  of  silver  cyanid  is  produced.  When  silver 
nitrate  is  added  to  an  alkaline  solution  of  a  cyanid,  the 
precipitate  which  at  first  forms  redissolves  on  stirring  and  a 
soluble  double  cyanid  (AgCN,KCN  or  AgCN,NaCN,  depending 
upon  the  alkali  used)  is  formed,  and  when  all  of  the  cyanid 
has  been  taken  up,  the  further  addition  of  silver  nitrate  causes 
a  decomposition  of  this  soluble  double  salt  and  the  formation 
of  a  permanent  precipitate  of  silver  cyanid.  Therefore,  the 
first  appearance  of  this  precipitate  affords  a  delicate  proof  of 
the  completion  of  the  reaction. 

These  equations  illustrate  the  reactions: 

2NaCN  +  AgNOs  =  AgCN,NaCN  -f-NaNOa. 

Double  cyanid 
of  silver  and  sodium 

AgCN,NaCN  +  AgNOs  =  2  AgCN  +  NaNOa. 

Silver  cyanid 

According  to  these  equations  it  is  seen  that  the  end- 
reaction  is  reached  when  two  molecules  of  the  alkali  cyanid 
have  reacted  with  one  molecule  of  silver  nitrate.  The  slightest 
excess  of  silver  nitrate  above  this  quantity  brings  about  a 
decomposition  of  the  double  salt  and  a  precipitation  of  the 
silver  cyanid,  as  above  stated. 

This  double  combination  is  so  firm  that  if  the  estimation 
is  done  in  the  presence  of  a  halogen,  no  permanent  precipitate 


132      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

of  silver  halid  is  formed  until  after  all  of  the  cyanogen  present 
has  been  converted  into  a  double  salt.  This  fact  is  taken 
advantage  of  in  the  processes  for  hydrocyanic  acid  and  alkali 
cyanid  in  which  potassium  iodid  is  employed  as  indicator 
in  the  presence  of  ammonia  water.  The  latter  prevents  the 
precipitation  of  silver  cyanids  and  thus  allows  the  silver  iodid 
to  precipitate  alone. 

N 
I  mil  of  —  AgNOs  V.S.  =0.005204  gm.  CN; 

0.005404  gm.  HCN; 

0.0098  gm.  NaCN; 

C.0130  gm.  KCN. 

Titration  with  Standard  Silver  Solution,  Using  Chromate 
Indicator— Vielhaber's  Method.  This  method  is  especially 
recommended  for  the  assay  of  weak  solutions  containing 
hydrocyanic  acid,  as  bitter  almond  oil,  bitter  almond  water, 
cherry  laurel  water,  etc.,  but  it  may  also  be  employed  for 
alkaline  cyanids. 

A  sufficient  quantity  of  an  aqueous  suspension  of  mag- 
nesium hydroxid  *  to  make  the  solution  opaque  and  distinctly 
alkaline  is  added;  this  is  followed  by  a  few  drops  of  potassium 

N     .  . 

chromate  indicator  and  then  the  —  silver  nitrate  delivered 

10 

into  the  mixture  from  a  burette  until  a  permanent  red  tint 
appears,  as  in  the  titration  of  haloid  salts.  The  method  is 
a  very  satisfactory  one  if  chlorids  are  absent. 

The  reactions  in  this  method  are  the  same  as  in  the  fore- 
going, but  the  end-reaction  (the  production  of  silver  chromate) 
does  not  occur  until  the  double  cyanid  is  completely  decom- 

*  Calcined  magnesia  triturated  with  water. 


ANALYSIS  BY  PRECIPITATION  133 

posed,  at  which  point  the  addition  of  another  drop  of  silver 
solution  reacts  with  the  chromate  and  produces  the  red  pre- 
cipitate (silver  chromate). 

The  equations  are  as  follows:  Sodium  is  used  in  the 
equations  instead  of  magnesium  in  order  to  make  the  explana- 
tion clearer. 

(a)  2NaCN + AgNOs  =  AgCN,NaCN + NaNOs  =  (2HCN) ; 

(b)  AgCN,NaCN + AgNOs  =  2  AgCN  +  NaNOs. 

These  equations  show  that  it  requires  two  molecules  of 
silver  nitrate  to  completely  precipitate  two  molecules  of  cyanid. 
169.89  gms.  of  AgNOs  is  equivalent  to  27.01  gms.  of  HCN, 
while  by  Liebig's  method  169.89  gms.  of  AgNOs  is  equivalent 
to  54.02  gms.  of  HCN. 

N 
I  mil  —  AgNOs  V.S.  =  0.002602  gm.  CN; 

0.002702  gm.  HCN; 

0.004902  gm.  NaCN; 

0.0065 1 1  g"^-  KCN. 

Example.  1.35  gms.  of  the  diluted  acid  is  mixed  with 
enough  water  and  magnesia  to  make  an  opaque  mixture  of 
about  10  mils.  Add  to  this  2  or  3  drops  of  potassium  chromate 
solution  and  then  from  a  burette  deliver  the  decinormal  silver 
nitrate  V.S.  until  a  red  tint  is  produced  which  does  not  dis- 
appear by  shaking. 

This  method  is  exemplified  in  the  U.S. P.  IX  Assay  of  Oil 
of  Bitter  Almonds. 

Titration  with  Standard  Silver  Solution,  Using  Potassium 
lodid  as  Indicator.  This  method,  employed  in  the  Assay  of 
Dilute  Hydrocyanic  Acid,  U.S.P.  IX,  is  conducted  as  follows: 

Into  a  loo-mil  flask  containing  25  mils  of  distilled  water 
and  5  mils  of  potassium  hydroxid  T.S.,  which  has  been  tared, 


134      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

pour  about  5  mils  of  the  acid,  and  weigh  it  accurately.  Then 
add  3  drops  of  potassium  iodid  T.S.  and  titrate  with  tenth- 
normal silver  nitrate  V.S.  to  the  production  of  a  slight  per- 
manent precipitate. 

Each  mil  of  tenth-normal  silver  nitrate  V.S.  used,  corre- 
sponds to  0.005404  gm.  of  HCN.  The  slight  precipitate  which 
is  produced  in  this  assay  and  which  marks  the  end  reaction, 
is  silver  iodid  (Agl). 

The  precipitate  of  silver  iodid  does  not  appear  until  the 
cyanid  has  been  entirely  converted  into  the  double  salt 
(AgCN.KCN). 

Estimation  of  Potassium  Cyanid  (KCN  =  65 . 1 1 ) .     O  ne  gm . 

of  potassium  cyanid  is  dissolved  in  sufficient  distilled  water 

to  make  100  mils,  then  65.11  mils  of  this  solution  mixed  with 

5   mils  of  ammonia  water  and  3   drops  of  potassium  iodid 

N 
solution  are  titrated  with  —  AgNOa  V.S.  until  the  appearance 

of  a  permanent  precipitate.     Each  mil  indicates  2  per  cent. 
I  mil  of  —  AgNOs  V.S.  =  0.0130  gm.  KCN. 

Estimation  of  Silver  SaSts 

Soluble  silver  salts  may  be  estimated  by  direct  titration 
with  standard  sodium  chlorid,  the  process  being  exactly  the 
converse  of  the  precipitation  methods  for  halogens.  The  stand- 
ard sodium  chlorid  solution  is  added  to  the  solution  of  the 
silver  salt  until  precipitation  ceases;  or  the  titration  may  be 
done  in  the  presence  of  chromate  indicator,  the  end-point 
being  then  known,  to  be  reached  when  the  red  color  of  the 
silver  chromate  disappears.  The  first  of  these  methods  is  im- 
practicable, too  much  time  being  consumed  in  waiting  for 
the  precipitate  to  settle  so  as  to  render  the  supernatant  liquid 


ANALYSIS  BY  PRECIPITATION  135 

sufficiently  clear  to  recognize  whether  a  precipitate  is  produced 
in  it  by  the  further  addition  of  the  standard  solution. 

If  chromate  indicator  is  used,  the  end-point  is  easily  over- 
stepped, because  of  the  slow  decomposition  of  the  silver 
chromate  by  the  chlorid.  It  is  better  to  add  an  excess  of 
sodium  chlorid  solution  and  then  retitra,te  with  standard  silver 
nitrate  solution  until  the  red  color  appears. 

Silver  salts  may  also  be  titrated  by  means  of  standard 

sulphocyanate  solution,  using  ferric  alum  as  indicator. 

N 
Assay  of  Silver  Nitrate  by  Means  of  —  Sulphocyanate. 

This  method,  as  applied  to  the  assay  of  halogen  compounds, 
is  described  in  the  preceding  pages.  The  great  advantage 
which  this  method  presents  over  the  others,  is  that  the  presence 
of  most  other  metals  does  not  interfere.  The  only  metal 
which  does  materially  interfere  with  the  determination  of 
silver  is  mercury. 

Example.  Weigh  accurately  about  0.8  gm.  of  the  silver 
salt,  and  dissolve  it  in  50  mils  of  distilled  water,  add  2  mils 
of  nitric  acid  and  2  mils  of  ferric-ammonium  sulphate  T.S. 
and  titrate  the  mixture  with  tenth-normal  potassium  sulpho- 
cyanate V.S.  until  a  permanent  reddish  color  of  ferric  sulpho- 
cyanate appears. 

The  following  equation  explains  the  reactions: 

AgNOs  +  KSCN  =  AgSCN  -f-  KNO3. 
10)169.89         10)97.18 

16.989  gms.        9.718  gms.  or  1000  mils  standard  V.S. 

Thus  each  mil  of  the  standard  V.S.  represents  l).o 1 6989 
gm.  of  pure  silver  nitrate,  or  0.010788  gm.  of  metallic  silver. 

Assay  of  Metallic  Silver  and  Silver  Alloys.  A  quantity 
of  the  metal,  weighing  about  0.5  gm.,  is  dissolved  in  10  mils 
of  nitric  acid,  and  after  complete  solution  is  attained  it  is 


136      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

heated  sufiiciently  to  drive  off  all  traces  of  nitrous  acid.  The 
solution  is  then  diluted  with  about  loo  mils  of  distilled  water 
and  assayed  by  one  of  the  methods  described  under  the  assay 
of  silver  nitrate.  The  sulphocyanate  method  is  the  preferred 
one. 

Assay  of  Silver  Oxid.  About  0.5  gm.  is  dissolved  in  3 
mils  of  nitric  acid  and  50  mils  of  distilled  water,  2  mils  of  ferric- 
ammonium  sulphate  T.S.  are  added  and  the  mixture  titrated 
with  a  standard  sulphocyanate  solution. 

Assay  of  Mercuric  Salts  by  Direct  Titration  with  Sulpho- 
cyanate Solution.  This  method  is  applicable  to  most  mercuric 
compounds  in  the  absence  of  halogens  (especially  chlorin). 
It  consists  in  titrating  the  mercuric  salt  in  the  presence  of 
nitric  acid  and  a  large  volume  of  distilled  water  with  a  standard 
sulphocyanate  solution. 

Ferric  ammonium  sulphate  is  used  as  indicator.  The  assay 
is  carried  out  as  follows : 

Assay  of  Mercuric  Oxid.  Dissolve  an  accurately  weighed 
quantity  (about  0.5  gm.)  of  the  mercuric  oxid  in  10  mils  of 
distilled  water  and  5  mils  of  nitric  acid.  Then  further  dilute 
the  solution  with  150  mils  of  distilled  water.  To  this  add  2 
mils  of  ferric-ammonium  sulphate  T.S.  and  titrate  with  tenth- 
normal potassium  sulphocyanate  V.S.  to  the  production  of  a 
permanent  yellowish-red  color.  Each  mil  of  the  sulphocyanate 
solution  used  corresponds  to  0.01083  gm.  of  HgO. 

HgO  +  2HN03  +  2KCNS  =  Hg(CNS)2  +  2KN03+H20. 

This  process,  while  not  strictly  a  precipitation  analysis,  is  for 
the  sake  of  convenience  included  under  this  heading.  The 
mercuric  sulphocyanate  formed  in  the  process  is  so  sparingly 
soluble  in  water  that  if  a  high  dilution  of  the  salt  being  titrated 
is  not  made,  a  heavy  white  precipitate  would  be  produced. 


ANALYSIS  BY  PRECIPITATION 


137 


TABLE  OF  SUBSTANCES  ESTIMATED  BY  PRECIPITATION 


Name. 


Formula. 


Molecular 
Weight. 


Standard 
Solution 

Used. 


Factor.* 


Acid,  hydrobromic 
"  hydrochloric . 
"     hydrocyanic . 


hydriodic 


AUyl-iso-thiocyanate  . 

Ammonium  bromid  , , 

"  chlorid . . 

"  iodid  .  .  . 

Calcium  bromid  .... 

"        chlorid 

Ferrous  bromid 

"        iodid 

Lithium  bromid  .... 

Potassium  bromid  . . . 

"  chlorid  .  .  . 


cyanid 


HBr 
HCl 
HCN 

HCN 

HCN 

HI 

:SNC3H 

NH^Br 

NH4CI 

NHJ 

CaBr2 

CaClj 

FeBra 

Fel2 

LiBr 

KBr 

KCl 

KCN 
KCN 
KCN 


80.93 

36.47 
27.02 

27.02 

27.02 

127.93 

99.12 

97.96 

53-50 

144.96 

199 -93 
I I I. 01 
215.66 
309 . 66 

86.86 
119.02 

74.56 

65.11 
65.11 
65.11 


AgNOa 


N 

ID 

N 
-AgN03 

N 
-AgN03 

without 
indicator 

N 
-AgN03 

with  chromate 
indicator 

N 
-AgNO, 

with  iodid 
indicator 

N 


AgNOa 


N 
-AgN03 

without 
indicator 

N 
-AgN03 

with  chromate 
indicator 

N 
-AgN03 

with  iodid 
indicator 


o . 008093 

0.003647 
0.005404 

0.002702 

0.005404 

0.012793 

0.004956 
0.009796 
0.00535 
0.014496 

o . 009996 

0.00555 

0.010784 

0.015484 
0.008686 

o. 01 1902 

0.007456 

0.013022 
o . 0065 I I 
0.013022 


*  This  is  the  coefficient  by  which  the  number  of  cc.  used  of  the  decinormal  solu- 
ion  is  to  be  multiplied  in  order  to  obtain  the  quantity  of  pure  substance  in  the 
imple  analyzed.  It  represents  the  weight  of  the  substance  precipitated  by  i  cc. 
"  the  decinormal  solution. 


^ 


THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 


TABLE  OF  SUBSTANCES  ESTIMATED  BY  PRECIPITATION- 

Continued 


Name. 


Formula. 


Molecular 
Weight. 


Standard 

Solution 

Used. 


Factor.* 


Potassium  iodid 

"          sulphocyanate 
Silver  (metallic) 

"      nitrate 

"      oxid 

Sodium  bromid 

"        chlorid 

"       iodid 

Strontium  bromid 

iodid 

Zinc  bromid 

"     chlorid 

"     iodid 


KI 
KSCN 

Ag2 


AgN03 
Ag^O 

NaBr 

NaCl 
Nal 

SrBr2 
Sri  2 

ZnBrj 

ZnClz 
Znio 


166.02 
97.18 
2X107.88 


169.89 
231.76 

102 .92 

58.46 
149.92 
247.47 

341-47 
225  .21 
136.29 
319.21 


N 

-  AgNOa 


10 

N 


NaC!  or 


N 


KSCN 


N 


AgNOg 


0.016602 
0.009718 
0.010788 


0.016989 
O.OII588 

0.010292 

0.005846 
0.014992 

0.012373 
0.017573 

O.OII260 
0.006814 
0.015960 


CHAPTER  X 
ANALYSIS  BY  OXIDATION  AND  REDUCTION 

An  extensive  series  of  analyses  are  made  by  these  methods 
with  extremely  accurate  results;  in  fact,  the  results  are  generally 
more  accurate  than  those  obtained  by  gravimetric  methods. 

The  principle  involved  is  exceedingly  simple.  An  oxidizing 
agent  is  employed  for  the  estimation  of  an  oxidizable  sub- 
stance, and  likewise  a  reducing  agent  is  employed  for  the 
estimation  of  a  reducible  substance.  Oxidizing  agents  are 
always  reducible  and  reducing  agents  always  oxidizable.  An 
oxidation  and  a  reduction  take  place  at  the  same  time,  i.e., 
the  oxidizing  agent  is  itself  reduced  in  the  operation  and  the 
reducing  agent  is  at  the  same  time  oxidized. 

Thus  substances  which  are  capable  of  absorbing  oxygen 
or  are  susceptible  of  an  equivalent  action  may  be  accurately 
estimated  by  subjecting  them  to  the  action  of  an  oxidizing 
agent  of  known  power,  and  from  the  quantity  of  the  latter 
required  for  complete  oxidation  the  weight  of  the  oxidizable 
substance  is  ascertained. 

Example.  Ferrous  oxid  (FeO),  an  oxidizable  substance, 
is  ever  ready  to  take  up  oxygen,  while  potassium  permanganate 
and  potassium  dichromate  are  always  ready  to  give  up  some 
of  their  oxygen.  When  potassium  permanganate  gives  up 
its  oxygen  in  this  way  it  is  reduced  and  decolorized,  while 
the  ferrous  oxid  in  taking  up  oxygen  is  oxidized  to  ferric  oxid 
(Fe203)>  The  decolorization  of  the  permanganate  here  spoken 
of  is  taken  advantage  of  in  volumetric  analysis  for  the  deter- 
mination of  the  completion  of  the  oxidation.  The  perman- 
ganate in  the  form  of  a  standard  solution  being  slowly  delivered 

139 


140       THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

from  a  burette,  until  it  is  no  longer  decolorized,  the  iron  salt 
is  known  to  be  completely  oxidized,  when  the  permanganate 
is  no  longer  reduced.    The  reaction  is  as  follows: 

ioFeO  +  2KMn04  =  5Fe203  +  2MnO  +K2O. 

Ferrous  oxid  Ferric  oxid 

The  oxidation  of  ferrous  oxid  by  potassium  dichromate 
is  shown  by  the  following  equation: 

6FeO  +  KsCrsOy  =  aFcsOa  +  CrsOs  +  K2O. 

As  before  stated,  an  oxidation  is  always  accompanied  by 
a  reduction,  the  oxidizing  agent  being  itself  reduced  in  the 
operation.  As  shown  in  the  above  equations,  the  manganic 
compound  is  reduced  to  a  manganous,  and  the  chromic  to 
a  chromous,  while  the  ferrous  salt  is  oxidized  to  a  ferric 
condition. 

In  the  same  way  any  substance  which  readily  yields  oxygen 
in  definite  quantity  or  is  susceptible  of  an  equivalent  action 
which  involves  its  reduction  to  a  lower  quantivalence,  may 
be  estimated  by  ascertaining  how  much  of  a  reducing  agent 
of  known  power  is  required  for  its  complete  reduction. 

Example.  The  available  chlorin  in  bleaching  powder  may 
be  accurately  ascertained  by  treating  it  with  a  standard  solu- 
tion of  arsenous  oxid,  and  from  the  volume  of  the  solution 
required  for  the  complete  reduction  of  the  chlorin,  the  quantity 
of  the  latter  present  is  found,  or  in  other  words,  from  the 
quantity  of  arsenous  oxid  (AS2O3),  oxidized  to  arsenic  oxid 
(AS2O5)  the  weight  of  the  chlorin  present  is  ascertained. 

The  principle  substances  which  are  used  as  oxidizing 
agents  in  volumetric  analysis,  are  potassium  permanganate, 
potassium  dichromate  and  iodin.  The  latter  contains  no 
oxygen,  but  it  abstracts  hydrogen  from  accompanying  water 
and  liberates  the  oxygen  which  does  the  oxidizing,  hence  iodin 
is  known  as  an  indirect   oxidizing  agent.     The  other  two 


ANALYSIS  BY  OXIDATION  AND  REDUCTION        141 

contain  available  oxygen  which  they  readily  give  up  when 
brought  in  contact  with  an  oxidizable  substance. 

The  principal  reducing  agents  or  deoxidizers  which  are 
used  in  volumetric  analysis  are,  sodium  thiosulphate,  sul- 
phurous acid,  oxalic  acid,  arsenous  oxid,  stannous  chlorid, 
ferrous  oxid,  hydriodic  acid,  hydrosulphuric  acid,  metallic 
zinc,  and  magnesium. 

/N\ 
Preparation  of  Decinormal  (  —  I  Potassium  Permanganate 

N 
(2KMn04  =  3i6.o6;  —  V.S.  =  3.i6o6  gms.  in  i  liter).  Abso- 
lutely pure  potassium  permanganate  cannot  be  obtained, 
therefore  the  preparation  of  a  decinormal  solution  of  "this 
salt  cannot  be  effected  by  simply  dissolving  the  requisite 
proportion  of  the  molecular  weight  in  the  water.  The  presence 
of  oxidizable  matter  in  the  water  used,  the  contact  of  dust 
and  exposure  to  light,  have  a  tendency  to  decompose  the 
salt  and  hence  weaken  the  standard  solution.  It  is  therefore 
advisable  to  use  boiling  distilled  water,  and  to  preserve  the  solu- 
tion in  amber  glass  bottles,  provided  with  ground-glass  stoppers. 
It  will  then  retain  its  strength  for  several  weeks,  but  should 
nevertheless  be  checked  by  titration  immediately  before  using. 
It  is  not  necessary,  and  it  is  usually  undesirable,  to  make 
the  solution  an  exact  decinormal  one.  It  is  preferable  to 
fix  the  titer  of  the  solution  and  employ  it  as  it  is. 

Place  3.5  gms.  of  pure  crystallized  potassium  perman- 
ganate in  a  flask,  add  1000  mils  of  distilled  water,  and  boil 
until  the  crystals  are  dissolved;  put  a  plug  of  absorbent 
cotton  in  the  mouth  of  the  flask  and  set  it  aside  for  two  days 
so  that  any  suspended  matter  may  deposit.  After  the  lapse 
of  this  time  pour  off  the  clear  solution  into  a  glass-stoppered 
bottle,  and  when  wanted  for  use  standardize  by  either  of  the 
following  methods: 


142       THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

Standardization  hy  Means  of  Iron,  Thin  annealed 
binding-wire,  free  from  rust,  is  one  of  the  purest  forms  of  iron.* 
O.I  gm.  of  such  iron  is  placed  in  a  flask  which  is  provided 
with  a  cork  through  which  a  piece  of  glass  tubing  passes, 
to  the  top  of  which  a  piece  of  rubber  tubing  is  attached, 
which  has  a  vertical  slit  about  one  inch  long  in  its  side,  and 
which  is  closed  at  its  upper  end  by  a  piece  of  glass  rod  (this 
arrangement  is  known  as  the  ''  Bunsen  Valve  ").  (See  Fig. 
41.)  Diluted  sulphuric  acid  is  added  and^gentle  heat  applied. 
The  iron  dissolves  and  the  steam  and  liberated  hydrogen  escape 
through  the  slit  under  slight  pressure.  The  air  is  thus  pre- 
vented from  entering  and  the  ferrous  solution  protected  from 
oxidation. 

A  better  form  of  apparatus  in  which  to  dissolve  the  iron 
and  avoid  oxidation  through  admission 
of  air  is  shown  in  Fig.  42.  A  loo-mil 
flask  is  fitted  with  a  rubber  stopper  and 

a  I  I  shaped  glass  tube;    into  this 

flask  is  placed  20  mils  of  diluted  sulphuric 
acid  (1:5)  and  then  2  or  3  crystals  of 


Fig.  41. 


Fig.  42. 


pure  sodium  carbonate;    this  causes  an  evolution  of  carbon 
dioxid  which  expels  the  air  from  flask.     The  o.i  gm.  of  iron 


*  This  contains  99.6  per  cent  of  iron. 


uMLirunniiA    C0LLE8E 

of   PHARMACY  . 

ANALYSIS  BY  OXIDATION  AND  REDUCTION        143 


wire  above  described  is  now  introduced,  the  stopper  inserted, 
and  a  beaker  containing  a  solution  of  pure  sodium  carbonate 
placed  in  position  so  that  the  tube  will  dip  into  the  solution. 
Gentle  heat  is  applied  until  the  iron  is  wholly  dissolved,  and 
only  a  few  minute  particles  of  carbon  remain  (which  must 
not  be  mistaken  for  iron).  When  the  flame  is  withdrawn 
the  cooling  of  the  flask  and  contents  causes  a  drawing  up 
of  the  sodium  carbonate  solution,  but  the  first  drops  that 
enter  the  flask  cause  an  effervescence  with  evolution  of  carbon 
dioxid,  which  drives  the  liquid  back  and  at  the  same  time 
fills  the  flask  with  the  gas;  this  is  repeated  until  the  flask 
and  contents  are  cold.  Another  useful  form  of  apparatus 
for    this    purpose    is    shown    in 

Fig.  43- 

When  the  iron  is  completely 
dissolved  a  small  quantity  of 
cold,  recently  boiled,  distilled 
water  should  be  used  to  rinse 
the  lower  end  of  the  stopper 
and  the  neck  of  the  flask,  and 
the  titration  with  potassium  per- 
manganate at  once  begun  and 
continued  until  a  faint  permanent 
pink  color  is  produced.  If  the 
solution  is  decinormal,  exactly 
17.84  mils  will  be  required  to  pro- 
duce this  result. 

The  iron  is  converted  by 
the  sulphuric  acid  into  ferrous 
sulphate,  Fe2  +  2H2SO4  =  2FeS04  +  2H2.  This  ferrous  sulphate 
is  easily  oxidized  by  the  air,  and  therefore  it  is  directed  that 
access  of  air  should  be  prevented,  and  the  distilled  water  with 


Fig.  43- 


1 


144       THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

which  the  solution  is  diluted  previously  boiled  in  order  to  drive 
off  any  dissolved  free  oxygen. 

ioFeS04  +  2KMn04  +  8H2SO4 

100)558.2       100)316.06  ^ 

5.582  gms.       3.1606  gms.  or  looo  mils  —  V.S. 

=  5Fe2(S04)3  +  K2S04  +  2MnS04+8H20. 

N 

This  equation,  etc.,  shows  that  each  mil  of  —  perman- 
ganate represents  0.005582  gm.  of  metallic  iron. 

Standardization  by  Means  of  Oocalic  Acid.  0.12605  gm. 
of  the  pure  crystallized  acid  is  weighed  (or  20  mils  of  deci- 
normal  oxalic  acid  carefully  measured)  and  placed  in  a  flask 
with  3  mils  of  sulphuric  acid  C.  P.  and  distilled  water  to  make 
100  mils.  The  solution  is  warmed  to  85°  C.  and  the  perman- 
ganate solution  delivered  in  from  a  burette. 

The  action  is  in  this  case  less  decisive  and  rapid  than  in 
the  titration  with  iron,  and  more  care  should  be  used.  The 
color  disappears  slowly  at  first,  but  afterwards  more  rapidly. 

Note  the  number  of  mils  of  the  permanganate  solution 
used,  and  then  dilute  the  remainder  so  that  equal  volumes 
of  it  and  of  decinormal  oxalic  acid  solution  will  exactly  corre- 
spond. 

Example.  Assuming  that  18.5  mils  of  the  permanganate 
solution  are  required  to  produce  a  permanent  pink  tint  in  the 
above  test,  then  the  permanganate  solution  must  be  diluted  with 
distilled  water  in  the  proportion  of  18.5  mils  of  the  permanganate 
solution  and  1.5  mils  of  water,  or  1850  mils  to  150  mils. 

After  dilution  a  new  trial  should  be  made,  in  which  50 
mils   of   the    diluted    permaCnganate    solution    should   require 

N 
exactly  50  mils  of  —  oxalic  acid  V.S. 


I 


ANALYSIS  BY  OXIDATION  AND  REDUCTION        145 

The  reaction  between  potassium  permanganate  and  oxalic 
acid  is  illustrated  by  the  following  equation: 

2KMn04  +  5(H2C204.2HoO)  +3H2SO4 

=  K2SO4  +  2MnS04  +  10CO2  +  18H2O. 

Standardization  hy  the  lodometric  Method,  This 
method,  which  was  proposed  by  Volhard,  is  the  most  accurate 
and  rapid  for  the  standardization  permanganate.  It  is  based 
upon  the  fact  that  potassium  permanganate  reacts  with  potas- 
sium iodid  in  solutions  acidulated  with  either  hydrochloric  or 
sulphuric  acid,  and  liberates  an  equivalent  quantity  of  iodin, 
which  may  be  estimated  by  standard  solution  of  sodium 
thiosulphate.    The  reactions  are  illustrated  by  the  equations 

{a)     2KMn04+8H2S04  +  ioKI 

316.06 

=  2MnS04+6K2S04+8H20  +  5l2; 

1269.2 

))      I2  +  2  (Na2S203 .  5H2O)  =  2NaI  +  Na2S406  +  10H2O. 


253-84  496.44 

Thus  it  is  seen  that  2KMn04  (316.06  gms.)  containing 

e  atoms  of  available  oxygen,  has  the  power  of  liberating 

.  equivalent  of  iodin,  i.e.,   10  atoms  or   1269.2   gms.    (see 

uation  a)   and  that  496.44  gms.   of  sodium  thiosulphate 

11  reduce  253.84  gms.  of  iodin   (see  equation  h).    Hence 

N 
mils  of  —  sodium  thiosulphate  (containing  25.384  gms.) 

1  reduce,   and  therefore  be  equivalent  to   12.692   gms.   of 
in,    which   in    turn   represents   3.1606    gms.    of   potassium 

N 
jrmanganate.     Therefore  i  mil  of  the  —  thiosulphate  repre- 

mts  0.012692  gm.  of  iodin  and  0.0031606  gm.  of  potassium 
jrmanganate,  which  latter  is  the  quantity  of  potassium  per- 

N 
langanate  present  in  i  mil  of  its  —  V.S. 


146      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

The  process  is  conducted  as  follows:  Into  a  200-mil  flask 
place  about  0.5  gm.  of  potassium  iodid  and  10  mils  of  diluted 
sulphuric  acid,  add  to  this  (slowly  from  a  burette)  exactly 
10  mils  of  the  permanganate  solution  to  be  standardized  and 
dilute  the  mixture  (which  is  brown  in  color,  because  of  the 
liberated  iodin)  with  distilled  water  to  about  150  mils.  Then 
slowly  titrate  (with  constant  stirring)  with  an  accurately  stand- 

N 
ardized  —  sodium  thiosulphate  until  the  color  of  the  solution 

is  a  faint  yellow,  then  add  a  few  drops  of  starch  solution  and 
continue  the  titration  until  the  color  is  discharged.  Note  the 
number  of  mils  consumed  and  dilute  the  permanganate  with 
distilled  water  so  that  equal  volumes  of  the  two  solutions 
correspond  to  each  other. 

Example.  If  13  mils  of  the  thiosulphate  solution  were 
required,  then  each  io  mils  of  the  permanganate  solution  must 
be  diluted  to  13  mils. 

Standardization  ivith  Ferrous  Aminoniufn  Sulphate 
{Mohfs  salt)  (FeS04-  (NH4)2S04-6H20).  392.14  gms.  of  this 
salt  contain  55.82  gms.  of  iron  (3.512  gms.  contain  0.5  gm.  of 
iron).  3.512  gms.  of  the  salt  are  accurately  weighed  out  and 
dissolved  in  sufficient  recently  boiled  distilled  water  to  make 
250  mils.  Fifty  mils  of  this  solution  containing  o.i  gm.  of 
iron  are  transferred  to  a  small  flask,  10  mils  of  diluted  sul- 
phuric acid  added,  and  then  the  permanganate  solution  to 
be  standardized  is  run  in  slowly  until  a  faint  pinkish  tint 
appears.  Whatever  number  of  mils  is  consumed  th  t  number 
represents  o.i  gm.  of  iron,  and  must  be  diluted  to  17.91  mils 
to  make  the  solution  exactly  decinormal. 

Volumetric   Analyses  by  Means  of  Potassium  Permanganate 

When  potassium  permanganate  solution  is  added  to  a  solu- 
tion of  any  readily  oxidizable  substance  strongly  acidulated 
with  sulphuric  acid,  it  undergoes  reduction,  as  shown  in  the 


ANALYSIS  BY  OXIDATION  AND  REDUCTION        147 

equation  below.  The  molecule  (2KMn04)  has  eight  atoms 
of  oxygen  which  it  gives  up  in  the  process  of  oxidation.  These 
eight  atoms  of  oxygen  unite  with  the  replaceable  hydrogen 
of  an  accompanying  acid,  liberating  an  equivalent  amount 
of  acidulous  radical.  Three  of  these  atoms  of  oxygen  liberate 
sufficient  acidulous  residual  to  combine  with  the  potassium 
and  manganese  of  the  permanganate,  while  the  other  five 
atoms  are  available  for  direct  oxidation. 

2KMn04  +  3H2SO4 -=  K2SO4  +  2MnS04  f  3H2O  +  5O, 

or,  for  combination  with  the  hydrogen  of  more  acid,  more 
acidulous  residual  being  set  free,  to  combine  with  the  salt 
acted  upon. 

2KMn04  +  8H2S04=K2S04  +  2MnS04+8H20  +  5(S04). 

5(804)  when  combined  with  ioFeS04  forms  Feio(S04)i5 
or  5Fe2 (804)3,  ferric  sulphate.  Thus  it  is  seen  that  one 
molecule  of  potassium  permanganate  (2KMn04)  has  the 
power  of  .converting  10  molecules  of  a  ferrous  salt  to  the 
ferric  state. 

The  equation  in  full  is 

loFeSOi  +  2KMn04  +  8H2SO4 

-K2S04  +  2MnS04+8H20  +  5Fe2(S04)3. 

We  have  seen  that  2KMn04  has  5  atoms  of  oxygen 
available  for  oxidizing  purposes,  and  that  each  of  these  will 
combine  with  2  atoms  of  hydrogen.  2KMn04  is  consequently 
chemically  equivalent  to  10  atoms  of  hydrogen,  and  a  normal 
solution  of  this  salt  when  used  as  an  oxidizing  agent  is  one 
that  contains  in  one  liter  one-tenth  of  the  weight  of  2KMn04 
expressed  in  grams,  and  a  decinormal  solution,  one  which 
contains  one-hundredth  of  this  weight. 

As  before  stated,  when  potassium  permanganate  is  brought 


k 


148       THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

in  contact  with  a  ferrous  salt  or  other  oxidizable  substance, 
it  is  decomposed  and  decolorized.  Hence  when  titrating  with 
a  standard  solution  of  this  salt  it  is  decolorized  so  long  as  an 
oxidizable  substance  is  present.  As  soon,  however,  as  the 
oxidation  is  completed  the  standard  solution  retains  its  color 
when  added  to  the  substance,  and  the  first  appearance  of  a 
faint  red  color  is  the  end-reaction,  and  the  oxidation  is  known 
to  be  completed. 

In  titrating  with  potassium  permanganate  it  must  be 
remembered  that  excess  of  free  acid  (preferably  sulphuric) 
should  always  be  present  in  the  solution  titrated,  in  order 
to  keep  the  resulting  manganous  and  manganic  oxids  in  solu- 
tion; these,  forming  a  dense  brown  precipitate,  would  make 
it  difficult  if  not  quite  impossible  to  recognize  the  pinkish 
color  of  the  end-reaction.  Sulphuric  acid  alone,  if  in  large 
excess,  has  a  reducing  effect  upon  potassium  permanganate. 

Nitric  and  hydrochloric  acids  are  prejudicial  and  should 
be  avoided;  they  are,  however,  frequently  present  in  salts 
which  are  to  be  analyzed,  and  in  such  event  should  be 
removed  by  converting  them  into  sulphate.  By  adding  a 
small  excess  of  sulphuric  acid  and  applying  heat,  until  hydro- 
chloric acid  or  nitrous  vapors  are  no  longer  evolved,  the 
chlorid  or  the  nitrate  is  converted  into  sulphate,  and  the 
deleterious  effect  of  their  presence  overcome.  Hydrochloric 
acid,  unless  present  in  very  small  quantities,  and  the  titra- 
tion conducted  at  a  low  temperature,  will  vitiate  the  analysis 
through  its  action  upon  the  permanganate  whereby  chlorin 
is  liberated,*  thus 

KMn04  +  8HC1  =^  KCl  4-  MnCl2  +  4H2O  +  5CI. 

*  This  decomposition  of  the  permanganate  by  hydrochloric  acid  is  due 
to  the  presence  of  ferric  salt,  which  latter  seems  to  act  catalytically,  for  oxalic 
acid  may  be  accurately  titrated  with  permanganate  even  in  the  presence  of 
hydrochloric  acid,  no  chlorin  being  given  off.  Thus  the  decomposition  o^ 
the  permanganate  is  not  due  to  the  hydrochloric  acid  alone. 


ANALYSIS  BY  OXIDATION  AND  REDUCTION        149 

A  very  convenient  way  of  obviating  the  irregularities  due 
to  the  presence  of  hydrochloric  acid  is  to  add  a  few  grams 
of  manganous  sulphate  *  to  the  solution  before  titrating  it. 

Mercuric  sulphate  J  and  magnesium  sulphate  may  also 
be  used  with  satisfactory  results. 

Potassium  permanganate,  being  so  readily  decomposed  by 
contact  with  organic  matter,  should  be  protected  from  such 
contact.  It  should  never  be  filtered  through  paper  (glass- 
wool  or  guncotton  may  be  used),  nor  should  it  be  used  in 
a  Mohr's  burette  or  in  any  other  apparatus  in  which  it  is 
in  contact  with  rubber  or  cork.  Furthermore,  all  substances 
of  an  oxidizing  or  reducing  nature,  aside  from  that  being 
analyzed,  must  be  excluded  from  the  solution.  Among  such 
substances  may  be  mentioned  hydriodic  acid,  sulphureted 
hydrogen,  nitrous  acid  and  the  lower  oxids  of  nitrogen,  phos- 
phorous and  hypophosphorous  acids,  thiosulphuric,  sulphurous, 
and  all  the  other  acids  of  sulphur  except  sulphuric,  also  ous 
salts  and  the  metallic  suboxids  and  peroxids. 

Burettes  and  other  apparatus  which  have  been  used  for 


*  Kessler  and  Zimmermann  suggest  using  20  mils  of  a  solution  of  man- 
ganous sulphate  (200  gms.  per  liter). 

X  Cady  and  Ruediger  (J.  A.  C.  S.,  XIX,  575)  concluded  from  the  following 
general  principles  that  it  is  possible  to  titrate  iron  with  permanganate  in  the 
presence  of  hydrochloric  acid  if  an  excess  of  mercuric  sulphate  be  added  to 
the  solution.  Mercuric  halids  in  solution  ionize  to  an  extremely  slight  extent, 
while  the  mercuric  salts  of  oxyacids  are  readily  ionized,  since  compounds  of 
slight  ionization  always  result  when  their  constituent  ions  meet;  mercuric 
halids  are  always  produced  when  a  mercuric  salt  of  an  oxyacid  is  added  to  a 
solution  containing  halogen  ions.  Therefore  when  mercuric  sulphate  solution 
and  hydrochloric  acid  are  mixed,  ionization  of  both  occurs,  and  the  mercuric 
ions  unite  with  the  chlorin  ions  and  produce  mercuric  chlorid  which  is  only 
very  slightly  ionized.  In  the  presence  of  a  large  excess  of  mercuric  sulphate, 
the  mercuric  ions  resulting  from  its  dissociation  diminish  the  ionization  of 
the  mercuric  chlorid  until  it  is  practically  nil.  Thus  no  chlorin  ions  will  be 
present  in  the  solution  to  induce  decomposition  of  the  permanganate. 


150        THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

permanganate,  should  be  emptied  and  rinsed  immediately 
after  use,  and  any  manganic  oxid  which  may  be  adhering 
to  the  glass  should  be  removed  by  means  of  hydrochloric 
acid  and  boiled  water. 

Not  only  oxidizable  substances  but  reducible  substances 
may  be  estimated  by  means  of  potassium  permanganate. 

In  the  estimation  of  oxidizable  substances  the  standard 
potassium  permanganate  is  added  directly  to  the  acidulated 
solution  of  the  substance  being  .analyzed.  The  completion  of 
the  oxidation  being  then  known  by  the  appearance  of  a  faint 
pinkish  tint.     This  is  the  direct  method. 

In  the  estimation  of  reducible  substances  (i.e.,  oxidizing 
substances)  the  indirect  or  the  residual  method  is  employed. 

In  this  an  accurately  weighed  or  measured  quantity  of 
the  substance  is  brought  together  with  an  excess  of  a  third 
substance  having  reducing  power,  and  which  is  similarly 
effected  by  the  permanganate  and  by  the  substance  analyzed. 
After  completion  of  the  reaction  the  excess  of  the  reducing 
substance  is  found  by  titration  wdth  standard  permanganate. 
The  difference  between  the  quantity  so  found  and  that  originally 
added  gives  the  quantity  which  reacted  with  the  salt  under 
analysis,  and  from  this  the  calculation  is  made. 

On   the  Use  of   Empirical    Permanganate   Solutions. 

A.  If  the  standardization  of  the  solution  is  done  by  means 
of  iron,  as  described  on  page  142,  o.i  gm.  of  iron  wire  (repre- 
senting 0.0996  gm.  of  pure  iron)  will  require  17.84  mils  of  the 
permanganate  solution  if  the  latter  is  exactly  decinormal. 
If  less  than  this  quantity  of  solution  is  used  (say  17.5  mils) 
it  indicates  that  the  solution  is  stronger  than  decinormal, 
and  may  either  be  diluted  so  that  each  17.5  mils  will  measure 
17.84,  or  it  may  be  used  as  it  is.     This  latter  is  in  most  cases 


1VNALYSIS  BY  OXIDATION  AND  REDUCTION       151 

preferable.     The  value  of   i   mil  of  the  solution  in  iron  is 
calculated  thus : 

17.5  mils.  :  I  mil:  10.0996  gm.  :  x.        :x;=o.oo569  gm. 

If  a  solution  of  this  strength  is  to  be  used  for  the  estima- 
tion of  iron,  simple  multiplication  of  the  number  of  mils  used 
by  0.00569  gm.  gives  the  weight  of  Fe  present.  If,  however, 
this  solution  is  employed  for  the  titration  of  other  oxidizable 
substances,  the  number  of  mils  consumed  is  multiplied  by 
0.00569  gm.  and  then  by  a  fraction  in  which  the  numerator 
represents  a  quantity  of  the  substance  examined,  equivalent 
in  grams  to  an  atom  of  iron  in*  its  reaction  with  permanganate, 
and  the  denominator  is  the  atomic  weight  of  iron. 

Example.  If  in  a  titration  we  use  40  mils  of  a  perman- 
ganate solution,  the  titer  of  which  has  been  found  to  be  i  mil 
=  0.00569  gm.,  the  calculation  would  be: 

in  the  case  of  ferrous  sulphate  (FeS04,  151.89), 

40X0.00569  gm.X^p^=o.6i92  gm.; 

in  the  case  of  oxalic  acid  (H2C204.2H20  =  126.05), 

63 
40X0.00569  gm.X—— =  0.256  gm.; 

in  the  case  of  hydrogen  dioxid  (H202  =  34), 

17 
40X0.00569  gm.X—— =0.0693  gm. 

B.  Another  way.  The  solution  just  mentioned,  of  which 
17.50  mils  are  consumed  in  titrating  o.i  gm.  of  iron  wire,  is 
compared  with  a  true  decinormal  permanganate  solution,  of 
which  17.84  mils  are  consumed  in  the  same  reaction.     The 


152      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

strength  of  the  former  solution  is  therefore as  compared 

with  a  decinormal  solution. 

In  titrating  with  this  solution  the  number  of  mils  consumed 

T  '7S/I 

are  to  be  multiplied  by  — —  and  then  by  the  true  decinormal 

factor  for  the  substance  being  analyzed. 

Example.     Forty  mils  of  the  solution  are  consumed. 

In  the  case  of  ferrous  sulphate  (FeS04=  151.89),  the  deci- 
normal factor  (i.e.,  the  weight  of  ferrous  sulphate  represented 
by  I  mil  of  a  true  decinormal  solution)  is  0.015 189  gm. 

1984 
40 X — —  X0.015189  gm,  =  0.6192  gm. 

/  D 

In  the  case  of  oxalic  acid  (H2C2O4,  2H20  =  126.05),  the  deci- 
normal factor  is  0.0063  8°^- 

1784  . 

40X— — X0.0063  gm.  =  o.256  gm. 

In  the  case  of  hydrogen  dioxid  (H202  =  34),  the  decinormal 
factor  is  0.001688  gm. 

1784 
40  X X0.0017  gm.  =  0.0693  ^^' 

C.  If  the  standardization  is  done  by  means  of  oxalic  acid, 
as  described  on  page  144,  in  which  10  mils  of  a  strictly  deci- 
normal oxalic  acid  solution  are  titrated  with  the  permanganate 
solution  which  is  being  standardized,  exactly  10  mils  of  the 
latter  will  be  consumed  if  it  is  of  decinormal  strength.  If 
in  the  trial,  however,  it  is  found  that  only  9.6  mils  are  con- 
sumed it  indicates  that  the  solution  is  stronger  than  decinormal; 

its  strength  beins:  expressed  by  —7-.    If,  on  the  other  hand, 


ANALYSIS  BY  OXIDATION  AND  REDUCTION       153 

more  than  lo  mils  of  the  solution  are  consumed  (say  10.4  mils) 

100 
the  solution  is  below  decinormal  strength,  namely, . 

In  using  a  solution  of  the  first  strength  the  number  of  mils 

100 
of  it  consumed  in  any  titration  is  to  be  multiplied  by  —7-  and 

then  by  the  decinormal  factor  for  the  substance  examined. 
In  the  case  of  the  weaker  solution  the  number  of  mils  con- 

100 
sumed  is  multiplied  by  —  and  then  by  the  decinormal  factor 

for  the  substance  being  analyzed. 

Examples.  Ferrous  sulphate  (FeS04=  151.89)  is  titrated 
with  the  stronger  solution,  40  mils  of  the  latter  being  consumed. 

100 
Then  40 X— 7-  X0.015189  gm. =0.628  gm. 

Oxalic  acid  (H2C2O4  •  2H2O  =  126.05),  4^  ^i^s  are  consumed. 

100 
Then  40 X— 7- X 0.0063  gm.  =  0.260  gm. 

Hydrogen  dioxid  (H202  =  34),  40  mils  are  consumed. 

rr^i  100 

Then  40 X—r-X 0.0017  gm.  =  0.0703  gm. 

D.  If  the  checking  of  the  permanganate  solution  is  done 
by  the  iodometric  method  (page  145)  and  it  is  found  that 
10  mils  of  the  permanganate  requires  the  use  of  13  mils  of 
decinormal    thiosulphate    solution,    the   titer   of   the   solution 

is  expressed  with  reference  to  decinormal  as   — .    In  using 

a  solution  of  this  strength,  the  number  of  mils  of  it  consumed 

13 
in  an  analysis  is  multiplied  by  —  and  then  by  the  decinormal 

factor  for  the  substance  analyzed. 


154      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 
TYPICAL  ANALYSES    WITH  PERMANGANATE 

A.  Direct  Titrations 

a.  Estimation  of  Ferrous  Sulphate  (FeS04H-7H20  =  278). 
One  gm.  of  ferrous  sulphate  is  dissolved  in  25  mils  of  water 
and  the  solution  strongly  acidulated  with  sulphuric  acid.  Deci- 
normal  potassium  permanganate  is  then  deUvered  from  a 
burette  until  a  permanent  pink  tint  is  obtained,  indicating 
the  complete  oxidation  of  the  ferrous  salt. 

The  reaction  is  as  follows  : 

(ioFeS04  +  7H2O)   +  2KMn04  +  8H2SO4 

100)  2  7*80         100)316.06  ^ 

27.80  gms.     =     3.1606  gm.  =  1000  mils  —  V.S. 

N 
2.780  gms.     =  100  mils  —  V.S. 

10 

N 
0.0278  gm.     =       •  I  mil    —  V.S. 

10 

=  5Fe2(S04)3  +  K2SO4  +  2MNSO4  +  78H2O. 

Thus  316.06  gms.  of  permanganate  =  2780  gms.  of  crys- 
tallized ferrous  sulphate,  which  equals  55.82  gms.  of  metallic 

N 
iron.     One  mil  of  —  permanganate  solution   therefore  repre- 
sents 0.0278  gm.  of  FeS04  +  7H20  or  0.005582  gm.  of  Fe. 

N 

In  the  analysis  35  mils  of  the  —  permanganate  were  con- 
sumed. The  I  gm.  taken  then  contains  35X0.0278  =  0.973 
gm.  or  97.3  per  cent. 

If  it  is  desired  that  each  cc.  of  the  permanganate  solution 
should  represent  a  certain  percentage  of  pure  salt,  a  molecular 
quantity  of  the  salt  should  be  taken  for  analysis  instead  of 
I  gm.  For  example,  if  2.78  gms.  be  taken,  each  mil  of  the 
decinormal  solution  consumed  will  correspond  to  i  per  cent, 
because  2.78  gms.  is  the  weight  of  crystallized  ferrous  sul- 


ANALYSIS  BY  OXIDATION  AND   REDUCTION        155 

phate  which  can  be  oxidized  by  loo  mils  of  the  decinormal 
solution.  If  half  of  this  weight  be  taken,  i.e.,  1.39  gms.,  each 
mil  of  the  permanganate  solution  compound  will  represent 
2  per  cent  of  pure  salt. 

Granulated  Ferrous  Sulphate  (FeS04  +  7H20)  is  estimated 
in  the  same  way  as  the  foregoing,  and  should  correspond 
with  it  in  strength. 

Exsiccated  {Dried)  Ferrous  Sulphate.  This  salt  is  tested 
in  the  same  manner  as  the  other  two  sulphates.  It  contains 
a  larger  percentage  of  ferrous  sulphate  than  the  other  two, 
having  less  water  of  crystallization.  Its  composition  is  approx- 
imately FeS04  f  3H2O. 

.ioFeS04     +     2KMn04     +     8H2SO4 

100)1518.9  100)316.06 

15.189  gms.  3.1606  gms.  or  1000  mils  —  standard  solution. 

==  5Fe2(S04)3  +  K2SO4  +  2MnS04  +  8H2O. 

Each  cc.  of  the  standard  solution  represents  0.015189  gm. 
of  anhydrous  (real)  ferrous  sulphate. 

h.  Estimation  of  Metallic  Iron  in  Ferrum  Reductum.    Fer 

rum  reductum  (reduced  iron)  always  contains  besides  metallic 
iron  a  varying  quantity  of  oxid.     Therefore,  in  assaying  this 
preparation  a  method  must  be  employed  which  will  estimate 
the  iron  only,  which  is  present  as  metallic  iron.     This  may 
be  done  by  means  of  a  solution  of  mercuric  chlorid  which 
re9,cts  with  metallic  iron  only  and  not  with  the  oxid. 
•  The  method  is  as  follows : 
Introduce  into  a  loo-mil  flask  i  gm.  of  reduced  iron,  pre- 
viously well  triturated.     Add  10  gms.  of  powdered  mercuric 
chlorid  and  50  mils  of  boiling  distilled  water.     Boil  for  five 
minutes,  shaking  it  frequently,  then  fill  the  flask  to  the  loo-mil 
mark  with  recently  boiled  and  cooled  distilled  water. 
2HgCl2 + Fe2  =  2FeCl2  +  2Hg. 


156      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

Let  it  cool  to  room  temperature  and  again  fill  to  the  mark. 
Shake  well,  and  after  a  few  minutes  filter  and  collect  20  mils 
of  filtrate.  Add  to  this  20  mils  of  dilute  sulphuric  acid,  and 
immediately  titrate  with  tenth-normal  potassium  permanganate 
V.S.  until  a  permanent  pink  color  is  produced. 

Each  mil  of  the  permanganate  solution  represents  0.005582 
gm.  of  metallic  iron. 

loFe  SO4    +    2KMn04    +    8H2SO4 

100)558.2  100)316.06  N  • 

5.582  gms.        =  3.1606  gms.  =  1000  mils  —  V.S. 

0.005582  gm.     =  0.0031606  gm.=   I  mil   :j^  V.S. 

=  5Fe2(S04)3  +  K2SO4  +  2MnS04  +  8H2O. 

Titration  with  an  Empirical  Permanganate  Solution.  A 
solution  of  permanganate,  which  is  found  upon  standardiza- 
tion to  be  of  a  strength  in  which  i  mil  is  equivalent  to  0.00512 
gm.  of  Fe,  is  to  be  used. 

Each  mil  of  this  solution  is  equivalent  to  the  following 
quantities : 

FeS04 0.01393  gm. 

FeS04  +  7H2O 0.02549     " 

FeCOs 0.01062     '' 

FeCl2 0.01162     " 

Fe 0.00512     " 

c.  Estimation  of  Oxalic  Acid  and  Oxalates  with  Potassium 
Permanganate  Solution  (H2C2O4  + 21120  =  126.05;  H2C2O4 
=  90).  The  estimation  of  oxalic  acid  may  be  accurately 
made  either  by  neutralization  with  a  standard  alkali  or  by 
oxidation  with  standard  permanganate.  The  latter  method 
is,  however,  the  one  to  be  employed  in  the  case  of  oxalates. 

The  oxidimetric  estimation  of  oxalic  acid  is  carried  out 
as  follows: 


ANALYSIS  BY  OXIDATION  AND  REDUCTION       157 

One  gm.  of  the  acid  (accurately  weighed)  is  dissolved  in 
sufficient  water  to  make  loo  mils.  Of  this  solution,  lo  mils 
(representing  o.i^  em.  of  the  acid)  is  taken  for  analysis.  Two 
mils  of  ^tkrtcd  sulphuric  acid  are  added,  the  solution  is  heated 
to  between  40°  C.  and  60°  C,  and  keeping  it  at  about  this 
temperature,  is  titrated  with  decinormal  potassium  perman- 
ganate, agitating  constantly,  until  a  faint  rose  tint  marks 
the  completion  of  the  reaction. 

Each  mil  of  the  permanganate  solution  consumed  repre- 
sents 0.0063  gm.  of  crystallized  oxalic  acid. 

The  reaction  is  as  follows: 

5  (H2C2O4  +  2H2O)  -f  3H2SO4  +  2KMn04 

icx))63o.25  100)316.06  j^i 

6.30gms.=  3.1606  gms.  =  1000  mils  — V.S. 

=  K2SO4  +  2MnS04  +  18H2O  +  10CO2. 

Direct  Percentage  Titration,    0.63  gm.  of  crystallized  oxalic 

N 
acid  is  oxidized  by  100  mils  of  —  permanganate.    Therefore 

N 
if  0.63  gm.  of  the  acid  is  taken  for  analysis,  each  mil  of  — 

permanganate  will  represent  i  per  cent. 

Titrating  with  an  Empirical  Solution.  If  the  permanganate 
is  checked  with  iron,  we  take  into  consideration  that  2KMn04 
will  oxidize  10  atoms  of  iron  (558.2  parts),  and  on  the  other 
hand  5  molecules  of  oxalic  acid  (630.25  parts).  If  the  titer  of 
the  permanganate  be  found  on  experiment  to  be  i  mil =0.00569, 
whatever  number  of  mils  of  this  solution  is  consumed  is  to 

63 
be  multiplied  by  0.00569  and  then  by  — — . 

Example.    0.3  gm.  of  oxalic  acid  requires  for  oxidation 

40  mils  of  a  permanganate  solution  whose  titer  is  i  mil =0.00569 

gm.  Fe,  the  calculation  is  made  as  follows: 

63 
40X0.00569  gm. X  — g^ -0.256  gm. 


158      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

0.256  gm.  is  the  quantity  of  pure  crystallized  oxalic  acid 
present  in  the  0.3  gm.  taken  for  analysis.  This  is  85.3  per 
cent. 

0.256X100 


0-3 


85-3. 


If  the  standardization  of  the  permanganate  is  done  by 
means  of  a  decinormal  oxalic  acid,  or  by  the  iodometric  method, 
the  calculation  is  as  described  on  pages  152-153. 

Oxalates  are  estimated  in  the  same  manner;  a  much 
larger  quantity  of  sulphuric  acid  is,  however,  required.  This 
serves  to  hberate  the  oxalic  acid  from  its  combination. 

The  presence  of  precipitates  of  sulphates  of  calcium,  barium, 
or  lead  does  not  interfere  with  the  recognition  of  the  end- 
point. 

N 
Each  mil  of  —  potassium  permanganate  represents 

Oxalic  acid  anhydrous  (H2C2O4) o-oo45  gm- 

Oxalic  acid  crystallized  (H2C2O4  +  2H2O) . .  .0.0063    ** 

d.  Estimation  of  Hydrogen  Dioxid  and  Barium  Dioxid  with 
Standard  Potassium  Permanganate.  Hydrogen  dioxid  [Hydro- 
gen peroxid)  (H202  =  34).  Hydrogen  dioxid  and  potassium 
permanganate,  though  both  oxidizing  agents,  will,  when  mixed 
in  an  acid  solution,  reduce  each  other.  The  reaction  which 
occurs  is  probably  primarily  an  oxidation  of  the  H2O2  to  a 
higher  oxid  (H2O4  (?))  which,  however,  immediately  breaks 
up  with  the  liberation  of  oxygen.  The  method 'of  assaying 
hydrogen  dioxid  by  means  of  permanganate  is  applicable  not 
only  to  this  substance  but  also  to  the  estimation  of  barium 
dioxid  and  the  soluble  alkali  peroxids.  The  method  is  usually 
carried  out  by  adding  the  permanganate  solution  to  the  dioxid 
in  a  solution  acidulated  with  sulphuric  acid.  Immediate 
decolorization  of  the  permanganate  occurs,  as  long   as   any 


ANALYSIS  BY  OXIDATION  AND  REDUCTION        159 

hydrogen  dioxid  is  present.  When  the  latter  has  been  entirely 
taken  up  the  permanganate  is  no  longer  decolorized  and  a 
faint  pink  tint  marks  the  end-point.  In  the  estimation  of 
the  pharmacopoeial  or  commercial  dioxid  solutions,  containing 
2  or  3  per  cent  of  H2O2,  a  measured  quantity  is  taken  for 
analysis.  The  specific  gravity  of  the  solution,  being  nearly 
that  of  water,  i  cc.  is  taken  to  represent  i  gm.  In  the  case 
of  solutions  of  hydrogen  dioxid  of  high  percentage  strength, 
it  is  advisable  to  take  a  weighed  quantity  for  analysis.  If 
hydrochloric  acid  is  present  a  small  quantity  of  manganese 
sulphate  should  be  introduced  before  titrating. 

The  assay  is  conducted  as  follows : 

An  accurately  weighed  quantity  (about  2  gms.)  of  the  hydro- 
gen dioxid  solution  is  diluted  with  20  mils  of  distilled  water, 
and  acidulated  with  20  mils  of  diluted  sulphuric  acid  and  then 

N        ' 
the  —  permanganate  solution  run  in  from  a  burette,  stirring 

after  each  addition  until  a  permanent  faint  pink  tint  appears. 
The  reaction  is  as  follows : 

5H2O2     +     2KMn04     +     3H2SO4 

100)170  100)316.06  -^T 

1.7  gms.  3.1606  gms.  =  1000  mils  -—  permanganate  V.S. 

N 
o.ooi  7  gm.  •  =       I  mil  —  permanganate  V.S. 

=    K2SO4  +  2MnS04  +    8H2O  +    5O2. 

N 
Thus  each  mil  of  —  permanganate  represents  0.0017  g"^- 

of  absolute  hydrogen  dioxid.  Assuming  that  in  the  above 
estimation  35.5  mils  of  the  permanganate  solution  were  required, 
then  the  2  gms.  taken  for  analysis  contained  0.0017  gm. X35.5, 
which  is  0.06035  &^-  o^  absolute  H2O2.  This  corresponds  to 
3.017  per  cent. 


160      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

The  Direct  Percentage  Method.  Ten  gms.  of  the  solution  is 
diluted  with  water  to  measure  loo  mils.  Seventeen  mils  of  this 
diluted  solution  (containing  1.7  gms.  of  hydrogen  dioxid)  is  acid- 
ulated with  sulphuric  acid  and  titrated  with  decinormal  perman- 
ganate, as  above  described.  Each  mil  of  the  permanganate  solu- 
tion consumed  will  represent  o.i  per  cent  by  weight  of  H2O2. 

J'itraiion   with  an   Empirical  Solution.     A    permanganate 

solution  is  on  hand  which  is  found  upon  standardization  with 

iron  to  be  i  mil  =  0.00569  gm.  Fe.     To  use  this  solution  as 

it  is,  we  take  into  consideration  that  2 KMn04=  (316.06)  =  10 

atoms   of  Fe   (558.2)    and   also   5   molecules  of  H2O2  (170). 

31.606   gms.    KMn04,  =55-82    gms.    Fe,    =17    gms.    H202- 

Whatever  number  of  mils  of  this  permanganate  solution  is  used, 

17 
multiplied  by  0.00569  gm.  and    then  by  — —,  will  give  the 

weight  of  H2O2  present  in  the  sample  analyzed. 

Estimation  of  Volume  Strength.  Let  us  look  at  the  above 
equation  in  a  different  light. 

We  see  that  when  potassium  permanganate  and  hydro- 
gen dioxid  react,  10  atoms  of  oxygen  are  liberated. 

The  permanganate  itself  when  decomposed  liberates  five 
atoms  of  oxygen.  Therefore  of  the  above  ten  atoms  only 
five  come  from  the  hydrogen  dioxid. 

5H202  =  5H20  +  50; 
2KMn04  +  3H2S04  =  K2S04  +  2MnS04  +  3H20  +  50. 

In  order  to  find  the  factor  for  volume  of  available  oxygen, 
see  the  following  equation,  etc. : 

5H202  +  2KMn04  +  3H2S04=K2S04  +  2MnS04+8H20+50  +  50. 

100)316.06  -^T  100)80 

3.1606  gms.  or  1000  mils  —  V.S.=  0.80  gm. 

I  mil  —  V.S.=  0.0008  gm. 

10 


ANALYSIS  BY  OXIDATION  AND  REDUCTION       161 

N 
Thus  it  is  seen  that  each  mil  of  —  potassium  permanganate 

V.S.   represents  0.0008   gm.   of   oxygen.     But  we  require   to 
find  the  volume  of  oxygen,   not  the  weight  represented  by 

I  mil  of  —  permanganate. 

1000  mils  of  oxygen  at  0°  C.  and  760  mm.  pressure,  weigh 
1.43  gms.  Therefore,  if  1.43  gms.  measure  1000  mils,  0.0008 
gm.  will  measure  0.57  mil. 

The   factor,   then,   for  volume  of  oxygen  liberated  when 

.      N 
hydrogen  dioxid  is  titrated  with  —  potassium  permanganate, 

N 
is  0.57,  and  the  number  of  mils  of  the  —  potassium  perman- 
ganate consumed  in  the  titration  gives  the  volume  of  oxygen 
liberated  by  the  quantity  of  hydrogen  dioxid  taken. 

N 
Thus  if  iQ  mils* of  the  —  V.S.  were  required, 

0.57  X 19  =  10.83  "^^Is  of  oxygen. 

It  is  convenient  to  operate  upon  i  mil  of  hydrogen  dioxid 
solution.  Then  each  mil  of  potassium  permanganate  V.S. 
used  will  represent  0.57  mil  of  available  oxygen  and  is  neces- 
sary only  to  multiply  the  number  of  mils  by  this  factor  to 
find  the  volume  of  available  oxygen. 

If  any  other  quantity  than  i  mil  of  dioxid  be  taken  for 
analysis,  it  will  be  necessary  after  multiplying  by  0.57  to 
divide  the  result  by  the  quantity  of  dioxid  solution  taken,  in 
order  to  find  volume  strength. 

Hydrogen  dioxid  solution  may  also  be  volumetrically 
assayed  by  Kingzett's  method,  which  is  described  under 
lodometry. 

The  gasometric  estimation  is  also  described  further  on. 


162      THE   ESSENTIALS   OF   VOLUMETRIC  ANALYSIS 

Barium  Dioxid  {Barium  Peroxid)  (Ba02=  169.37).  This 
substance  is  assayed  by  treating  it  with  an  acid,  and  then 
estimating  the  Uberated  hydrogen  dioxid,  as  follows: 

Weigh  off  2  gms.  of  the  coarse  powder,  put  it  in  a  porcelain 
capsule,  add  about  10  mils  of  ice-cold  water,  then  7.5  mils 
of  phosphoric  acid  (85  per  cent),  and  sufficient  ice-cold  water 
to  make  25  mils.  Stir  and  break  up  the  particles  with  the  end 
of  the  stirrer  until  a  clear  or  nearly  clear  solution  is  obtained 
and  all  that  is  soluble  is  dissolved. 

Five  mils  of  this  solution  (which  corresponds  to  0.4  gm. 
of  barium  dioxid)  is  measured  off  for  assay. 

Drop  into  this  from  a  burette,  with  constant  stirring,  deci- 
normal  potassium  permanganate  until  a  final  drop  gives  the 
solution  a  permanent  pink  tint. 

About  40  mils  of  the  decinormal  permanganate  should  be 
required  to  produce  this  result. 

In  this  process,  the  first  step  is  the  forniation  of  hydrogen 
dioxid  by  treating  the  barium  dioxid  with  phosphoric  acid, 
as  illustrated  by  the  following  equation : 

Ba02   +  H3PO4  =   BaHP04   +  H2O2. 

169.37  54 

The  hydrogen  dioxid  is  then  estimated  with  decinormal 
permanganate,  as  described  above. 

5(Ba02)     =     5H2O2     +     2KMn04     +     3H2SO4 

100)845.85  100)170  100)316.06  -j^    • 

8.4585  gms.=  1.7  gms.  3.1606=1000'  cc.  —  permanganate. 

=  K2SO4  +  2MnS04  +  8H2O  +  5O2. 

Sodium  Perborate  (NaBOa  +4H2O  =  154-06).  Dissolve  an 
accurately  weighed  quantity  (about  0.25  gm.)  of  the  salt  in 
a  mixture  o-f  50  mils  .of  distilled  water  and  10  mils  of  diluted 
sulphuric   acid,    and   titrate   the   solution   with    tenth-normal 


ANALYSIS  BY  OXIDATION  AND   REDUCTION        163 

potassium   permanganate   V.S.    until   a   final   drop   gives   the 
solution  a  permanent  pink  color. 

Each  mil  of  the  standard  permanganate  used  corresponds 
to  0.0008  gm.  of  available  oxygen,  which  should  be  present 
in  not  less  quantity  than  9  per  cent.  The  reaction  involved  is 
represented  by  the  equation: 

NaB03-4H20+H2S04  =  NaHS04  +  B(OH)3+H202+H20. 

154.06 

Percarbonates  may  be  assayed  in  the  same  manner. 
Na2C206-H20  +  2H2S04=2NaHS04  +  2C02  +  H202+H20. 

B.  Residual  Titrations 

a.  Methods  in  which  an  Excess  of  Standard  Permanganate  is  Added 
AND  the  Excess  Determined  by  Residual  Titration  with  Stand- 
ard Oxalic  Acid. 

Estimation  of  Nitrous  Acid  and  Nitrites.  Nitrous  acid, 
when  brought  in  contact  with  a  potassium  permanganate  solu- 
tion acidulated  with  sulphuric  acid,  is  oxidized  to  nitric  acid. 
Two  molecules  of  KMn04  reacting  with  5  molecules  of  HNO2, 
as  the  equation  shows, 

5HNO2  +  2KMn04  +  3H2SO4 

-  5HNO3  +  K2SO4  +  2MnS04  +  3H2O. 

In  the  case  of  nitrites,  as  for  example  sodium  nitrite,  the 
oxidation  takes  place  in  the  same  manner,  and  the  process 
may  be  applied  with  equally  good  results  to  the  salts,  as  well 
as  to  free  HNO2.  At  ordinary  temperatures  the  oxidation 
proceeds  very  slowly,  but  at  a  temperature  of  40°  C.  (104°  F.) 
rapid  reaction  occurs.  But  because  of  the  volatility  of  nitrous 
acid  in  acidulated  solutions  of  its  salts  it  is  impossible  to 
accurately  estimate  them  by  direct  titration  with  permanganate 
at  a  raised  temperature. 


164      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

It  is  customary  to  add  the  nitrite  solution  to  a  measured 

volume  of  warm  acidulated  standard  permanganate  solution 

and  then  to  titrate  the  excess  of  permanganate  with  standard 

oxalic  acid  solution. 

N- 
The  assay  is  conducted  as  follows:  To  50  mils  —  potassium 

permanganate  V.S.  add  100  mils  of  distilled  water  and  5  mils 
of  sulphuric  acid.  To  this  solution  add  by  means  of  a  pipette 
10  mils  of  a  solution  of  sodium  nitrite  (i  gm.  in  100  mils),. 
In  adding  the  sodium  nitrite  solution  immerse  the  lip  of  the 
pipette  beneath  the  surface  of  the  permanganate  mixture.  Warm 
the  liquid  to  40°  C,  allow  it  to  stand  for  five  minutes  and  then 

N  .        .        •  . 

titrate  with  —  oxalic  acid  V.S.  until  the  color  of  the  perman- 
ganate solution  is  just  discharged.  Subtract  the  number  of 
mils  of  the  oxalic  acid  solution  used  from  the  number  of  mils 
of  permanganate  solution  taken,  and  multiply  the  remainder 
by  0.0034505  gm. 

5NaN02  +  2KMn04 +3H2SO4 
5X69.01=345.05 

=  5NaN03  +  K2S04  +  2MnS04+3H20. 

N 
Each  mil  of  —  potassium  permanganate  V.S.  corresponds 

to  0.0034505  gm.  of  NaN02. 

The  Assay  of  Hypo  phosphites  may  be  made  by  the  same 
method. 

An  accurately  weighed  quantity  of  hypophosphorous  acid 
or  its  salt  is  dissolved  in  water,  the  solution  strongly  acidulated 

N 
with  sulphuric  acid,  and  then  a  measured  excess  of  —  potas- 
sium permanganate  solution  added.    The  mixture  is  boiled 
for  fifteen  minutes  to  hasten  and  facilitate  the  oxidation  and 


ANALYSIS  BY  OXIDATION  AND  REDUCTION       165 

N 
then   the  excess  of  permanganate  solution  titrated  with  — 

[Oxalic  acid  V.S. 

Example.    Three  gms.  of  the  acid  are  diluted  with  water 

to  make  60  mils.     Of  this  solution  6  mils   (representing  0.3 

gm.  of  the  acid)   are  carefully  removed  with  a   pipette  and 

introduced  into  a  flask.     Three  mils  of  sulphuric  acid  are  added 

N 
and  the*  50  mils  of  —  potassium  permanganate  solution,  and 

the  mixture  boiled  for  fifteen  minutes. 

The  potassium  permanganate,  in  the  presence  of  sulphuric 
acid,  oxidizes  the  hypophosphorous  acid  to  phosphoric,  as 
the  equation  shows : 

5HPH2O2   +  6H2SO4  +  2(2KMn04) 

2)330.2  2)632.12 

100)165.1  100)316.06  -^T 

1.651  gms.  3.1606  gms.  or  1000  mils  —  V.S. 

=  5H3PO4  +  6H2O   +  fK2S04  +  4MnS04. 

Each  mil  of  the  decinormal  V.S.  represents  0.00165 1  gm. 
of  absolute  hypophosphorous  acid.  The  quantity  of  per- 
manganate solution  directed  to  be  added  is  slightly  in  ex- 
cess. The  excess  is  then  ascertained  by  retitration  with 
decinormal  oxalic  acid.  Each  mil  of  oxalic  acid  required  cor- 
responds to  I  mil  of  decinormal  permanganate  which  has 
been  added  in  excess  of  the  quantity  actually  required  for  the 
oxidation. 

The  excess  of  permanganate  colors  the  solution  red,  and 
the  oxalic  acid  V.S.  is  then  added  until  the  red  color  just 
disappears,  which  indicates  that  the  excess  of  permanganate 
is  decomposed. 

If  4.7  mils  of  decinormal  oxalic  acid  are  required,  it  indi- 
cates that  50  mils  -4.7  mils  =  45.3  mils  of  decinormal  perman- 


166      THE   ESSENTIALS   OF  VOLUMETRIC   ANALYSIS 

ganate  were  actually  used  up  in  oxidizing  the  hypophosphorous 
acid;  therefore 

0.001651  gm.X45.3  =  o.o747+  gm. 
of  absolute  hypophosphorous  acid,  HPH2O2,  or 

'    0.0747  X 100 

=  24.7  per  cent. 

•3 

In  this  process  boiling  facilitates  the  oxidation,  but  if  the 
acid  is  boiled  before  sufficient  permanganate  has  been  added 
to  completely  oxidize  it,  decomposition  will  take  place.  Hence 
direct  titration  with  permanganate  is  impossible  and  the  residual 
method  must  be  resorted  to. 


b.  Methods  Involving  a  Precipitation  by  Oxalic  Acid  and  the  Titra- 
tion OF  THE  Excess  of  the  Latter  with  Standard  Permanganate. 

Assay  of  Calcium  Carbonate  and  Soluble  Calcium  Salts. 

Dissolve  an  accurately  weighed  quantity  of  calcium  carbonate 

(0.3  to  0.4  gm.)  in  10  mils  of  distilled  water  and  10  mils  of 

diluted  hydrochloric  acid,  and  boil  the  solution  to  expel  all 

carbon  dioxid.     Transfer  this  solution  to  a  200-mil  graduated 

N 
flask,  add  100  mils  of  —  oxalic  acid  V.S.,  render  it  alkaline  with 
10 

ammonia  water,  shake  well  and  allow  it  to  stand  for  three 

hours  at  from  60°  to  70°  C.  or  overnight  at  room  temperature. 

Cool  the  mixture,  if  necessary  dilute  with  distilled  water  to  the 

200-mil  mark,  mix  it  well,  filter  through  a  dry  filter  -into  a  dry 

flask,  reject  the  first  20  mils  of  filtrate,  and  proceed  as  follows: 

To  100  mils  of  the  filtrate  (representing  half  of  the  calcium 

carbonate    taken)    add   diluted    sulphuric   acid   until   of   acid 

reaction,  then  add  25  mils  more  of  the  same  acid,  warm  the 

N 
solution  to  60°  or  70°  C,  and  titrate  with  —  potassium  per- 


ANALYSIS  BY  OXIDATION  AND   REDUCTION        IG/ 

manganate   V.S.   until   a   permanent    pale   pink  color  is  ob- 
tained. 

Deduct  the  number  of  mils  of  the  permanganate  solution 
used  (say  10.8  mils)  from  half  the  volume  of  the  oxalic  acid 
solution  taken  (50  mils)  and  multiply  the  remainder  (39.2 
mils)  by  0.005  gm.  This  gives  the  weight  of  CaCOa  in  half 
the  quantity  of  the  salt  taken  for  analysis. 

50 -10.8  =  39.2  mils; 

39.2X0.005  gm.  =0.1960  gm.; 

0.1960  X 100 
=  98  per  cent. 

Calcium  Oxid  (Calx),  CaO  =  56o7,  is  assayed  in  the  same 
manner. 

Estimation  of  Soluble  Calcium  Salts.  To  a  weighed  quan- 
tity of  the  calcium  salt  dissolved  in  water,  a  measured  excess 
of  normal  oxalic  acid  is  added'.  The  mixture  is  then^niade 
alkaline  with  ammonia  and  boiled,  to  facilitate  the  separa- 
tion of  the  precipitate.  It  is  then  cooled  and  diluted  with 
water  to  an  accurately  measured  volume,  and  after  filtra- 
tion  an  aliquot  portion  removed,   acidulated  with'  sulphuric 

N 
acid,  and  carefully  titrated  with  —  potassium  permanganate. 

Example.  0.4  gm.  of  calcium  chlorid  is  dissolved  in  water, 
'  10  mils  of  normal  oxalic  acid  added,  the  mixture  made  alkaline 
with  ammonia  water,  and  boiled  for  a  few  minutes.  It  is 
then  filtered,  the  residue  and  filter  washed  with  water,  and 
after  cooling  the  combined  filtrate  and  washings  are  diluted 
to  make  100  mils. 

Of  this  solution  50  mils  are  taken  for  analysis  (representing 
0.2  gm.  of  the  salt),  acidulated  with  sulphuric  acid,  and  then 

N         :  .  . 

titrated  with  —  potassium  permanganate  to  a  faint  rose  tmt. 


168      THE   ESSENTIALS   OF  VOLUMETRIC   ANALYSIS 

The  50  mils  of  solution  represent  5  mils  of  normal  oxalic 

acid,  which  is  equivalent  to  50  mils  of  decinormal  oxalic  acid, 

so  that  whatever  number  of  mils  of  decinormal  permanganate 

solution  is  required  in  the   titration,   that  quantity  is  to  be 

deducted  from  50  mils  and  the  difference  multiplied  by  the 

N 

—    factor  for  calcium  chlorid  to  find  the  quantity  of  pure 

CaCl2  present  in  0.2  gm. 

N 
If  14  mils  of  —  permanganate  are  employed,  then  14  from 

50  mils  leaves  36  mils,  the  quantity  of  decinormal  oxalic  acid 
solution  which  combined  with  the  0.2  gm.  of  calcium  chlorid. 
Then 

0.00555  gm.X36=o.i998  gm., 

the  quantity  of  pure  CaCl2  present  in  the  0.2  gm.,  or  99.9 
per  cent. 

Calcium  salts  to  be  estimated  by  this  method  must  be 
tolerably  pure,  and  free  at  least  from  impurities  which  would 
react  with  oxalic  acid  or  which  would  reduce  permanganate. 

Many  of  the  less  soluble  calcium  salts  may  be  estimated 
by  this  method,  but  they  must  be  subjected  to  longer  treat- 
ment with  the  oxalic  acid. 

Gold  and  lead  salts  may  also  be  estimated  by  the  same 
method. 

Estimation  of  Lead  in  the  Acetate  and  Subacetate.    Take 

for  assay  0.2  gm.  of  the  salt  or  2  gms.  of  the  solution  in  a 

beaker  and   add   20   mils   of  recently   boiled   distilled  water. 

Pour  this  slowly  and  with  constant  shaking  into  a  graduated 

.  «N  .        . 

cylinder  containing  50  mils,  of  —  oxalic  acid  V.S.     Wash  the 

beaker  with  small  portions  of  distilled  water  and  add  the 
washings  to  the  contents  of  the  cylinder.  Then  dilute  the 
mixture  to  100  mils  and  set  aside  to  allow  the  precipitate  to 


I  ANALYSIS  BY.OXIDATION  AND   REDUCTION         169 

ttle.  Remove  20  mils  of  the  clear  liquid  (representing  0.04 
gm.  of  the  salt  or  9.4  gni.  of  the  solution)  for  titration.  Add 
5  mils  of  (i  :  10)  sulphuric  acid,  warm  to  80°  C,  and  titrate 

N 
with  —  permanganate  until  a  final  drop  imparts  a  permanent 

pale  pink  tint. 

The  reactions  are  represented  by  the  following  equations: 


2HC2H3O2  +  2H2O. 


3(C2H302)2     + 
Lead  acetate 

H2G204-2H20 

325-1 

126.05 

=     PbC204 

+ 

320(C2H302)2 

Lead  subacetate 

+     2H2C204-2H20 

548.2 

2X126.05 

•    =     2PbC204 

+ 

2HC2Haae  ^  qHsO. 


H^|.5H. 


N 
Each  mil  of  —  oxalic  acid  represents  0.016257   gm.   of 

Pb(C2H302)2  or  0.013705  gm.  of  Pb20 (€211302)2,  or  0.010355 
gm.  of  Pb. 

Calculate  as  described  under  assay  of  calcium  carbonate. 

The  U.S.P.  IX  Method  is  essentially  the  same  as  the  fore- 
going.    It  is  as  follows : 

Assay  of  Lead  Acetate. '  Dissolve  an  accurately  weighed 
quantity  of  lead  acetate  (say  5  gms.)  in  sufficient  recently 
boiled  distilled  water  to  make  ico  mils  of  solution.  Mix 
10  mils  of  this  solution  with  5c  mils  of  tenth-normal  oxalic 
acid  V.S.  in  a  2co-mil  measuring  flask,  agitate  the  mixture 
thoroughly  for  five  minutes,  then  fill  the  flask  to  the  200-mil 
mark  with  distilled  water;  filter,  and  titrate  100  mils  of  the 
filtrate  (representing  one-twentieth  of  the  amount  of  lead 
acetate  originally  taken)  with  tenth-normal  potassium  per- 
manganate V.S.,  the  filtrate  being  previously  acidulated  with 
10  mils  of  sulphuric  acid  and  warmed  to  80°  C. 


170      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

Assay  of  Lead  Oxid  (PbO  =  223.10).  Dissolve  about  0.4 
gm.  of  lead  oxid  in  4  mils  of  acetic  acid  and  25  mils  of  recently 
boiled  distilled  water.  Add  50  mils  of  tenth-normal  oxalic 
acid  V.S.  and  dilute  with  distilled  water  to  make  200  mils  of 
fluid.  Mix  well  and  filter.  Collect  ico  mils  of  filtrate.  Acidu- 
late it  with  20  mils  of  diluted  sulphuric  acid,  warm  the  solution 
to  80°  C.  and  titrate  with  tenth-normal  potassium  perman- 
ganate V.S.  as  in  preceding  assay. 

Each  mil  of  the  oxalic  acid  V.S.  corresponds  to  con  15*5 
gm.  of  PbO. 

c.  Methods  Involving  a  Reduction  by  Means  of  Oxalic  Acid,  and 
Retitration  of  the  Excess  of  the  Latter  with  Potassium  Per- 
manganate. 

Estimation  of  Manganese  Dioxid  (Mn02).  The  estima- 
tion of  manganese  dioxid  depends  upon  the  fact  that  when 
it  is  boiled  with  oxalic  acid  in  the  presence  of  sulphuric  acid 
definite  reaction  takes  place,  as  the  equation  shows:  , 

Mn02+H2C204  +  H2S04  =  MnS04  +  2C02  +  2H20. 

In  the  estimation  a  measured  excess  of  oxalic  acid  solu- 
tion is  added,  together  with  some  sulphuric  acid,  and  the 
mixture  heated  until  solution  is  complete. 

The  excess  of  oxalic  acid  is  then  found  by  retitration  with 
standard  permanganate  solution.  It  is  well  to  use  a  normal 
oxalic  acid  solution  and  a  decinormal  permanganate  solution. 

0.5  gm.  of  the  dioxid  is  a  convenient  quantity  to  operate 
upon.  Each  mil  of  decinormal  solution  represents  0.004346 
gm.  of  Mn02. 

Example.  0.5  gm.  of  Mn02  is  treated  with  3  mils  of  sul- 
phuric acid  and  10  mils  of  normal  oxalic  acid  solution,  which 
is  equivalent  to  100  mils  of  decinormal  oxalic  acid  solution, 
the  mixture  is  heated  in  a  water- bath  to  80°  C.  and  then  treated 


ANALYSIS   BY  OXIDATION   AND   REDUCTION        171 

as  described  above,  and  upon  letitrating  25  mils  of  decinormal 
permanganate  are  required.     Thus 

100  mils  -25  mils  =  75  mils. 

N  .  .        . 

of  —  oxalic  acid  went  into  reaction  with  the  Mn02.     Then 
10 

75X0.004346  =  0.3259  gm. 

d.  Methods  Involving  a  Reduction  by  Means  of  a  Standardized 
Solution  of  a  Ferrous  Salt,  and  Titration  of  the  Remaining 
Unoxidized  Ferrous  Salt,  by  Permanganate. 

Estimation  of  Nitrates  (Pelouze).  This  method  consists 
in  adding  a  weighed  quantity  of  the  nitrate  to  an  acidulated 
solution  of  a  ferrous  salt  of  known  strength,  and,  when  reaction 
is  complete,  estimating  the  ferrous  salt  remaining,  by  titra- 
tion with  permanganate,  or  in  certain  cases  by  means  of 
dichromate  V.S.  The  principle  upon  which  the  method  is 
based  is^  that  when  nitric  acid  or  a  nitrate  is  brought  in  con- 
tact with  a  highly  acidulated  solution  of  a  ferrous  salt,  the 
former  gives  off  oxygen,  which,  passing  over  to  the  ferrous 
salt,  oxidizes  it  to  the  ferric  state,  while  at  the  same  time  NO 
is  evolved.     The  reaction  is 

2HNO3   +  6HC1   +  6FeCl2   =  3F2CI6   +  4H2O   +  2NO. 

Nitric  acid  Ferrous  chlorid 

126,02  760.44 

Iron 
334-92 

Thus  one  molecular  weight  of  nitric  acid  (63.01)  will 
oxidize  three  molecular  weights  of  ferrous  salt,  or  three  atoms 
of  iron  (167.46). 

Either  hydrochloric  or  sulphuric  acid  may  be  employed. 
The  former  is  preferred  by  most  operators,  and  it  is  generally 


172      THE  ESSENTIi\LS  OF  VOLUMETRIC  ANALYSIS 


agreed  that  in  order  to  attain  results  of  sufficient  precision 
the  estimation  should  be  done  in  the  presence  of  hydrochloric 
acid  only.  In  using  hydrochloric  acid,  however,  where  the 
titrations  are  to  be  made  with  permanganate,  certain  precau- 
tions (previously  mentioned)  must  be  observed,  because  of 
the  evolution  of  chlorin  which  will  otherwise  take  place  and 
spoil  the  analysis.  This  may  be  obviated  by  adding  to  the 
solution  to  be  titrated  an  excess  of  manganese  sulphate. 

The  NO  which  is  produced  during  the  reaction  must  be 
removed  by  boiling  before  titration  with  permanganate  is 
begun.  Air  must  be  absolutely  excluded  during  the  entire 
process  to  prevent  oxidation  of  ferrous  salt  by  the  atmospheric 
oxygen,  as  well  as  to  prevent  oxidation  of  NO  to  HNO3, 
which  will  oxidize  more  ferrous  salt.  The  exclusion  of  air 
may  be  partially  affected  by  the  use  on  the  flask  of  a  Bunsen 
valve  stopper  (see  Fig.  41),  but  the  best  method  is  to  employ 
an  apparatus  so  arranged  that  a  constant  stream  of  CO2  or 
H  gas  may  be  passed  through  it  (see  Fig.  44). 

This  method,  although  theoretically  perfect,  is  in  practice 
liable  to  great  irregularities,  and  will  give  fairly  good  results 
only. if  the  directions,  especially  those  as  to  exclusion  of  air, 


Fig.  44. 

are  faithfully  carried  out.    The  method  of  Kjeldahl  is  to  be 
preferred. 

To  conduct  the  process,  weigh  accurately  1.5  gms.  of  flower 


ANALYSIS  BY  OXIDATION  AND   REDUCTION       173 

wire*  free  from  rust  (the  iron  content  of  which  is  known), 
place  it  an  Erlenmeyer  flask  which  is  provided  with  a  double 
perforated  stopper  fitted  with  two  glass  tubes,  one  of  which 
should  reach  just  to  the  surface  of  the  Hquid  in  the  flask  when 
in  place,  and  the  other,  which  is  the  outlet  tube,  should  reach 
no  lower  than  the  bottom  of  the  stopper.  The  first  of  these 
tubes  is  connected  with  an  apparatus  generating  carbon  dioxid 
or  hydrogen,  while  the  outlet  tube  serves  to  convey  the  gas 
into  the  air  or  into  another  flask  containing  water  or  an  alka- 
line solution.  30  to  40  mils  of  pure  fuming  hydrochloric  acid 
are  added  to  the  iron  wire  in  the  flask,  gentle  heat  is  applied, 
and  a  stream  of  either  CO2  or  H  passed  through  the  flask  and 
maintained  throughout  the  entire  process.  When  the  iron  is 
completely  dissolved,  the  stopper  is  raised  just  long  enough 
to  introduce  a  small  glass  tube  open  at  one  end  and  con- 
taining the  nitrate  to  be  estimated.  The  quantity  or  nitrate, 
taken  must  be  equivalent  to  not  more  than  0.2  gm.  of  HNO3. 
The  stopper  is  then  reinserted,  heat  applied,  and  gradually 
increased  until  the  reaction  is  complete.  The  free  hydro- 
chloric acid  liberates  nitric  acid  from  the  nitrate  and  oxidation 
of  a  portion  of  the  iron  is  effected.  The  ferrous  chlorid  is 
oxidized  to  ferric  chlorid,  as  the  equation  shows,  and  the 
solution  becomes  at  first  dark  brown  through  the  presence 
of  NO.  As  the  heat  is  increased,  the  dark-brown  color  of 
the  solution  is  gradually  changed  to  yellow,  as  ferric  chlorid 
is  formed,  and  increases  in  intensity  until  the  reaction  is 
complete,  then  the  color  remains  stationary  and  indicates  com- 
pletion of  oxidation.  The  solution  is  now  allowed  to  cool, 
but  the  stream  of  CO2  or  H  gas  is  maintained.  Forty  mils  of 
a  solution  of  manganese  sulphate  are  now  added  (this  is  not 
necessary  if  sulphuric  acid  is  used  instead  of  hydrochloric), 

*  Or  fine  piano-forte  wire. 


174      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

.     N 
and  titration  with  —  potassium  permanganate  solution  begun, 

in  order  to  determine  the  quantity  of  unaltered  ferrous  salt 
remaining  in  the  solution.  Assuming  that  89  mils  were  re- 
quired, the  calculation  is  made  as  follows : 

Since  one  molecule  of  HNO3    (63.01)   reacts   with  three 
atoms  of  iron   (167.46)   the  quantity  of  iron  found  to  have 

63.01 
been  oxidized,  if  multiplied  by  ~~2~~~2j  will  give  the  quantity 

of  nitric  acid  present. 

Example.   1.5  gms.  of  iron  wire,  99.6  per  cent  Fe=  (1.494  gms. 

of  iron),  is  dissolved  in  hydrochloric  acid,  as  above  described, 

and  0.6  gm.   of  potassium  nitrate,   KNO3    (loi.ii),  added. 

After  oxidation,  98  mils    of   decinormal   permanganate   were 

N 
required.    Each  mil  of  —  KMn04  =  0.005582  gm.  of  Fe. 
^  10 

0.005582  gm.X  98  =  0.547  gm.  of  oxidized  iron.     1.494  gms. 
of  iron  were  originally  taken. 
Therefore, 

1.494 
0-547 


Then 


o;m.  =  the  quantity  of  iron  oxidized. 
0.947  ^  ^  ^ 


0.047X6^.01 

^       ^      =0.357  gm.  of  HNO3, 


167.46 
which  equals 

0.947X63.01X101.1T  .  ^.^.^ 

V"'];x7^ =^0-571  gm.  of  KNO3, 

167.46X63.01  ^'     ^  ■*' 

or  95.5  per  cent  pure. 

N  N 

It  is  usually  advisable  to  use  an  -  instead  of  an  —  KMn04 

solution. 


ANALYSIS  BY  OXIDATION  AND   REDUCTION       175 

Assay  of  Chlorates.  The  chlorate  is  made  to  react  with 
a  ferrous  salt,  which  is  added  to  its  solution  in  excess,  and 
which  it  oxidizes  to  the  ferric  state.  The  unoxidized  ferrous 
salt  is  then  titrated  with  permanganate.  The  chlorate  present 
being  calculated  from  the  quantity  of  ferrous  salt  which  it 
oxidized.  The  method  is  applicable  to  the  assay  of  nitrates 
and  chromates  as  well. 

Dissolve  an  accurately  weighed  quantity,  say  0.5  gm.,  of 
potassium  chlorate  in.  sufficient  distilled  water  to  make  ico 
noils  of  solution.  Take  10  mils  of  this  solution  (representing 
0.05  gm.),  add  25  mils  of  acidulated  ferrous  sulphate  T.S.^ 
Place  the  whole  in  a  flask  provided  with  a  Bunsen  valve  (see 
Fig.  41)  and  boil  for  ten  minutes.  Now  cool  the  solution  and 
add  10  mils  of  manganous  sulphate  T.S.  and  titrate  the  residual 
ferrous  sulphate  with  tenth-normal  potassium  permanganate  V.S. 
At  the  same  time  conduct  a  parallel  experiment  with  another 
portion  of  10  mils  of  acidulated  ferrous  sulphate  T.S.  The 
difference  in  the  number  of  mils  of  the  permanganate  solution 
used  in  the  two  titrations,  multiplied  by  0.00204  gi^->  gives 
the  weight  of  KCIO3  present. 

The  reaction  is  expressed  as  follows : 

KCl03  +  6FeS04H-3H2S04  =  KCl+3Fe2(S04)3+3H20. 

122.56     6X151.91 

Example.  In  the  blank  experiment  25  mils  of  the  acidulated 
ferrous  sulphate  T.S.  required  26.2  mils  of  tenth-normal  per- 
manganate V.S. 

The  residual  ferrous  sulphate  in  the  actual  assay  required 
2.4  mils  of  the  permanganate  V.S.,  hence  23.8  mils  of  the  per- 


*  Dissolve  3  gms.  of  clear  crystals  of  ferrous  sulphate  in  90  mils  of  distilled 
tter,  then  add  sufficient  sulphuric  acid  to  make  100  mils  of  solution. 


176      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

manganate   V.S.   represent   the   chlorate   which   reacted   with 
the  ferrous  sulphate. 

0.00204X23.8X100 

=97- 1  per  cent. 

0.05  ^'     ^ 

Chromic  Acid  and  Chromates.  Chromic  acid  oxidizes 
ferrous  salts  in  the  same  manner  as  nitric  acid  does.  The  reac- 
tion is  thus  expressed:  • 

6FeS04  +  6H2S04  +  2Cr03  =  Cr2(S04)3+3Fe2(S04)3  +  6H2Q. 

SFe =334.92  200 

To  an  accurately  weighed  quantity  of  ferrous  ammonium 
sulphate  (Mohr's  salt)  FeS04  +  (NH4)2S04  +  6H20  (the  per- 
manganate titer  of  which  is  known),  which  is  dissolved  in  a 
sufficient  quantity  of  diluted  sulphuric  acid  in  an  Erlenmeyer 
flask,*  add  a  weighed  quantity  of  the  chromate  or  chromic 
acid  in  a  concentrated  aqueous  solution.  Warm  the  mixture 
on  a  water-bath,  under  a  constant  stream  of  carbon  dioxid 
until  the  liquid  assumes  a  clear  green  color.  This  occurs  in 
a  few  minutes,  and  indicates  complete  reduction  of  the  chromate. 

Now  allow  the  solution  to  cool,  continuing  the  passage 
of  carbon  dioxid  through  the  flask,  and  transfer  the  cold 
solution  to  a  large  beaker,  and  after  diluting  it  to  about  300 
cc.  and  strongly  acidifying  it  with  sulphuric  acid,  titrate  it 

N 
for  unoxidized  ferrous  salt  by  means  of  —  potassium  per- 
manganate. 

It  is  usually  sufficient  to  mix  the  solutions  cold,  but  it 
is  better   to   employ  heat  after  mixing.     A   large   excess   of 

*  This  flask  should  be  provided  with  a  stopper  having  two  perforations 
through  which  glass  tubes  are  passed,  one  of  these,  which  serves  to  convey 
carbon  dioxid  gas,  should  reach  close  to  the  surface  of  the  liquid,  the  other 
tube  should  end  just  below  the  stopper  and  serve  as  the  outlet  tube.  See 
Fig.  44. 


ANALYSIS  BY  OXIDATION  AND   REDUCTION       177 

"errous  salt  is  .unnecessary.     It  is  imperative  to  dilute  the 

lution  highly  before  titration,  as  then  only  can  the  end  color 

oint  be  accurately  determined  in  the  green  solution.     The 

se  of  an  excess  of  sulphuric  acid  before  titration  is  likewise 

emanded.     A  violet-red  color  marks  the  end-point,  and  unless 

00  great  a  quantity  of  chromate  be  taken,  or  the  solution  be 

sufficiently  diluted,  it  can  be  easily  recognized.     This  method 

applicable  not  only  to  free  chromic  acid  and  soluble  chro- 

mates,  but  also  to  chromates  which  are  insoluble  in  water.* 

It  can  therefore  be  employed  for  the  indirect  estimation  of 

such  bases  as  are  precipitable  by  chromic  acid,  out  of  neutral, 

ammoniacal  or  acetic  acid  solutions,   as   for   instance   lead, 

bismuth  and  barium. 

Finally,  the  method  may  be  employed  for  the  estimation 

of  chromic  oxids.     The  solution  of  the  latter  is  treated  with 

an  excess  of  sodium  carbonate,  bromine  water  added,  and 

heat  applied    until   a   clear   solution   results.     This   solution, 

which  contains  all  of  the  chromium  in  the  form  of  sodium 

chromate,  is  evaporated,  the  residue  dissolved  in  dilute  acetic 

acid  and  the  chromium  completely  precipitated  by  means  of 

lead  acetate.     The  precipitated  lead  chromate  is  then  treated 

as  above. 

The  calculation  is  made  with  reference  to  the  equation, 

in  which  it  is  shown  that  one  molecule,  loo  of  chromic  oxid 

(CrOs),  is  equivalent  to  three  molecules  (167.46)  of  metallic 

100 
iron.     The  quantity  of  iron  oxidized,   multiplied  by  -7 — 7, 

gives  the  weight  of  chromic  oxid  present,  and  from  this  its 
equivalent  in  potassium,  sodium,  lead,  bismuth  or  barium 
chromate  is  calculated. 

*  In  the  case  of  insoluble  chromates  the  salt  is  shaken  directly  with  the 
ferrous  solution,  and  the  mixture  more  highly  diluted,  and  more  strongly 
heated,  than  in  the  case  of  soluble  salts. 


178      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

In  the  case  of  potassium  dichromate  (K2Cr207)  one  mole- 
cule  (294.2)   is  equivalent  to  six  atoms   (334.92)   of  metallic 

iron.     The  quantity  of  iron  oxidized  is  multipUed  by  — ^*— . 

^  334-92 
Example.    To  1.5  gms.  of  ammonio-ferrous  sulphate  (con- 
taining 0.2142  gm.  Fe)  add  0.1241  gm.  of  K2Cr^07  (molecular 
weight  294.2),  and  after  complete  oxidation,  titrate  the  solu- 

N 
tion  with  —  KMn04  to  determine  the  quantity  of  unchanged 

ferrous  salt.  Thirteen  mils  are  required.  Each  mil  represents 
0.005582  gm.  of  Fe. 

Thus,  13X0.005582  gm.  =  0.0725  gm.,  the  quantity  of  iron 
which  was  not.  oxidized  by  the  dichromate.  This,  deducted 
from  the  quantity  of  iron  originally  added  (0.2142-0.0725 
=  0.1417  gm.),  gives  the  quantity  which  was  oxidized. 

Then, 

O.I4I7XIOO  „  r    ^     ^ 

— ^^f—^— =  0.08475+  gm.  of  CrOs 

or 

0.1417X  294.2 


334-92 


=  0.1247  gm.  of  K2Cr207. 


Example.  To  1.5  gms.  of  ammonio-ferrous  sulphate  (con- 
taining 0.2142  gm.  of  Fe)  add  the  precipitate  of  barium 
chromate  obtained  from  0.2491  gm.  of  BaCl2  +  2H20  (molec- 
ular weight  244.29)  and  after  complete  oxidation,  titrate  with 

N 

-^  permanganate.     7.8  mils  are  consumed,  thus  7.8X0.005582 

=  0.04353  gm.  the  quantity  of  unoxidized  iron  present.  Then 
0.2142  -0.04353  =  0.17057  gm.  of  iron  oxidized  by  the  barium 
chromate. 

o.i7057X244.29^^_  _  BaCl.  +  .H.O. 

167.46 


I 


ANALYSIS  BY  OXIDATION  AND   REDUCTION       179 


Volumetric  Analysis  by  Means  of  Potassium  Bichromate 

In  some  respects  the  dichromate  possesses  advantages  over 
permanganate : 

1.  It  may  be  obtained  in  a  pure  state. 

2.  Its  solution  does  not  deteriorate  upon  standing  as  does 
that  of  permanganate. 

3.  It  is  not  decomposed  by  contact  with  rubber  as  the 
permanganate  is,  and  may  therefore  be  used  in  Mohr's  burette. 
Its  great  disadvantage,  however,  is  that  when  used  in  the 
estimation  of  ferrous  sahs  the  end-reaction  can  only  be  found 
by  using  an  external  indicator.  The  indicator  which  must 
be  used  is  freshly  prepared  potassium  ferricyanid  T.S.,  a 
drop  of  which  is  brought  in  contact  with  a  drop  of  the  solu- 
tion being  tested,  on  a  white  slab,  at  intervals  during  the 
titration,  the  end  of  the  reaction  being  the  cessation  of  the 
production  of  the  blue  color,  when  the  two  liquids  are  brought 
together.  Thus  the  estimation  by  potassium  dichromate  is 
cumbersome,  and  very  exact  results  are  not  as  easily  obtained 
as  with  permanganate. 

Besides  ferrous  salts,  a  great  many  other  substances  may 
be  estimated  by  oxidation  analysis  with  dichromate.  Among 
them  nitrates,  sulphates,  arsenous  acid,  barium,  lead,  ferric 
salts  after  reduction  by  stannous  chlorid  or  an  alkaline  sul- 
phite, but  not  after  reduction  by  means  of  metallic  zinc 
The  presence  of  the  dissolved  zinc  salt  interferes  with  the 
reaction  of  the  ferricyanid  indicator.  Ferrous  salts  may  be 
estimated  in  the  presence  of  hydrochloric  acid,  by  means  of 
dichromate,  without  the  precautions  that  apply  in  the  case 
of  permanganate.  Chromium  as  chromate  may  be  indirectly 
estimated;  an  excess  of  a  solution  of  a  ferrous  salt  being 
added  and  then  the  excess  determined  by  dichromate.  lodids, 
thiosulphates  and  alkalies  may  also  be  estimated  by  means 
of  potassium  dichromate. 


180      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

N 
Preparation    of    Decinormal    —    Potassium    Dichromate 

N 
(K2Cr207  =  294.2;  —  V.S.  =  4.903  gms.  in  1000  mils). 

4.903  gms.  of  purq  potassium  dichromate  *  which  has 
been  pulverized  and  dried  at  120°  C.  is  dissolved  in  suffi- 
cient water  to  make  1000  mils  of  solution. 

It  will  be  noticed  that  ^  of  the  molecular  weight  of  the 
dichromate  (expressed  in  grams)  is  taken  in  the  preparation 
of  1000  mils  of  this  solution.  The  reason  for  this  is  that  one 
molecule  of  potassium  dichromate  when  treated  with  an  acid 
yields  three  atoms  of  nascent  oxygen  which  are  available  for 
oxidizing  purposes,  thus 

K2Cr207+4H2S04  =  K2S04  +  Cr2(S04)3+4H20  +  03; 

and  since  each  atom  of  oxygen  is  equivalent  to  two  atoms 
of  hydrogen,  one  molecule  of  the  dichromate  must  be  equiv- 
alent to  six  atoms  of  hydrogen.  Hence  a  normal  solution  of 
potassium  dichromate,  when  used  as  an  oxidizing  agent,  should 
contain  one-sixth  of  the  molecular  weight,  expressed  in  grams, 
in  1000  mils  (see  definition  for  normal  solution)  and  its  deci 
normal  solution  ^. 

If  a  standard  solution  of  potassium  dichromate  is  to  be 
made  for  use  as  precipitant,  as  in  the  titration  of  barium, 
one-fourth  of  the  molecular  weight  is  to  be  taken  for  1000 
mils  of  the  normal  solution,  as  explained  in  Chapter  III. 

Standard  solution  of  potassium  dichromate  is  sometimes 
used  as  a  neutralizing  solution  for  estimating  alkalies,  phenol- 
phthalein  being  used  as  indicator. 

When  used  for  this  purpose  the  normal  solution  con- 
tains   147. 1    gms.   in    I    liter  (one-half  the   molecular  weight 

*  Potassium  dichromate  for  use  in  volumetric  analysis  should  respond 
to  all  the  tests  for  purity  given  in  the  U.S.P.,  o"i-  it  should  be  recrystallized 
several  times  and  then  dried. 


ANALYSIS   BY  OXIDATION  AND   REDUCTION        181 

in  grams).      It   is    then   the   exact   equivalent  of  any  normal 
acid  V.S. 

2KOH  +  KsCraOr  =   2K2Cr04  +  H2O. 

2)112.2  2)294.2 

56.1  gms.         147. 1  gnis.,  or  1000  mils  normal  V.S. 

Decinormal  potassium  dichromate  may  also  be  used  in 
conjunction  with  potassium  iodid  and  sulphuric  acid  for 
standardizing  sodium  thiosulphate.  lodin  is  liberated  from 
potassium  iodid  in  this  reaction.  The  reaction  is  expressed 
by  the  equation 

K2Cr207  +  6KI  +  7H2S04  =  4K2S04  +  Cr2(S04)3  +  7H20  +  3l2. 

Thus  one  molecule  of  the  dichromate  will  liberate  six 
atoms  of  iodin,  therefore  a  normal  solution  should  contain 
one-sixth  of  the  molecular  weight,  and  a  decinormal  solution 
^  in  1000  mils.  The  solution  is  hence  of  the  same  strength 
as  that  which  is  used  for  oxidizing  purposes.  If  the  deci- 
normal solution  containing  14.71   gms.  in  i  liter  is  used,  it 

3N 
has  the  effect  of  a  —  solution. 
10 

The  decinormal  solution  which  is  used  as  an  oxidizing 
agent  is  chemically  equivalent  to  decinormal  potassium  per- 
manganate. When  used  for  the  purpose  of  liberating  iodin 
from  potassium  iodid,  it  is  the  equivalent  of  an  equal  volume 
of  decinormal  sodium  thiosulphate. 

Standard  potassium  dichromate  may  be  checked  in  the 
same  way  as  standard  permanganate,  with  pure  iron  wire. 

ESTIMATION    OF   FERROUS   SALTS   WITH   POTASSIUM   DICHROMATE 

I        For  titrating  ferrous  salts  the  decinormal  solution  of  dichro- 
mate is  used  in  the  following  manner: 
r— 


182      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

it  into  a  flask,  and  acidulate  it  with  sulphuric:  or  hydrochloric 
acid.  Now  add  gradually  from  a  burette  the  decinormal 
potassium  dichromate  until  a  drop  taken  out  upon  a  white 
slab  no  longer  shows  a  blue  color  with  a  drop  of  freshly 
prepared  potassium  ferricyanid  T.S.  Note  the  number  of 
mils  of  the  standard  solution  used,  multiply  this  number  by 
the  factor,  and  thus  obtain  the  quantity  of  pure  salt  in  the 
sample  taken. 

Ferrous  salts  strike  a  blue  color  with  potassium  ferricyanid, 
but  as  the  quantity  of  ferrous  salt  gradually  diminishes  during 
the  titration,  the  blue  becomes  somewhat  turbid,  acquiring 
first  a  green,  then  a  gray,  and  lastly,  a  brown  shade.  The 
process  is  finished  when  the  greenish-blue  tint  has  entirely 
disappeared. 

The  reaction  of  potassium  dichromate  with  ferrous  salts 
always  takes  place  in  the  presence  of  free  sulphuric  or  hydro- 
chloric acid  at  ordinary  temperatures.  Nitric  acid  should  not 
be  used. 

If  it  is  desired  to  estimate  ferric  salts  by  this  standard 
solution  it  is  necessary  to  first  reduce  them.  This  may  be 
done  by  metallic  magnesium,  sulphurous  acid,  the  alkali  sul- 
phites, or  by  stannous  chlorid. 

One  molecule  of  potassium  dichromate  yields,  under  favor- 
able circumstances,  3  atoms  of  oxygen.  This  is  shown  by 
the  following  equation : 

KaCrsOT  =  CrgOs  +  K2O  +  O3. 

Here  it  is  seen  that  the  three  liberated  atoms  of  oxygen 
combine  at  once  with  the  ferrous  oxid,  converting  it  into 
ferric  oxid: 

6FeO  +  03  =  Fe609     or    aFesOa. 

In  the  oxidation  of  a  ferrous  salt,  the  reaction  takes  place 
only  in  the  presence  of  an  acid. 


ANALYSIS  BY  OXIDATION  AND  REDUCTION       183 

The  dichromate  then  gives  up  its  oxygen.  Four  of  its 
oxygen  atoms  combine  at  once  with  the  replaceable  hydrogen 
of  the  accompanying  acid,  the  other  three  being  liberated. 
The  three  oxygen  atoms  thus  set  free  are  available  either  for 
direct  oxidation  or  for  combination  with  the  hydrogen  of 
more  acid.  In  the  latter  case  a  corresponding  quantity  of 
acidulous  radicals  is  set  free. 

KsCraOy  +  4H2SO4  =  K2SO4  +  ^2(804)3  +4H2O  +  O3. 

In  this  case  four  of  the  liberated  atoms  of  oxygen  combine 
with  eight  of  the  atoms  of  hydrogen  of  sulphuric  acid  and 
liberate  four  SO4  radicals,  which  at  once  combine  with  the 
K2  and  Cr2  of  the  dichromate.  The  other  three  atoms  are 
set  free.  If  seven  sulphuric  acid  molecules  are  used  instead 
of  four  molecules,  the  three  free  atoms  of  oxygen  will  liberate 
3(S04): 
K2Cr207  +  7H2SO4  =  K2SO4  +  Cr2(S04)3  +  7H2O  +  (804)3. 

If  this  liberation  of  3(804)  takes  place  in  the  presence  of 
a  ferrous  salt,  the  3(804)  will  combine  with  six  molecules  of 
the  ferrous  salt,  converting  it  into  a  ferric  salt: 

6FeS04  +  .s804-Fe6(S04)9  =  .3Fe2(SQ4)3; 

6FeS04  +  K2Cr207  +  7H28O4 

=  K28O4  +  Cr2(804)3  +  7H2O  +  [3Fe2(804)3]. 

If  in  the  above  case  hydrochloric  acid  is  used  instead  of 
sulphuric,  fourteen  molecules  of  the  former  must  be  taken  to 
supply  the  neccessary  hydrogen. 

The  seven  liberated  atoms  of  oxygen  must  have  fourteen 
atoms  of  hydrogen  to  combine  with. 

Three  of  these  atoms  of  oxygen  liberate  six  univalent  or 
three  bivalent  Acidulous  radicals. 


184      THE  ESSENTIALS   OF  VOLUMETRIC  ANALYSIS 

Therefore,  since  one  molecule  of  K2Cr^07  will  give  up 
for  oxidizing  purposes  three  atoms  of  oxygen,  which  are  equiva- 
lent chemically  to  six  atoms  of  hydrogen,  one-sixth  of  the 
molecular  weight  in  grams  of  the  dichromate,  dissolved  in 
sufficient  water  to  make  one  liter,  constitutes  a  normal  solution, 
and  one-tenth  of  this  quantity  of  K2Cr207  in  a  liter,  a  deci- 
normal  solution. 

Thus  the  estimation  of  ferrous  salts  is  effected  by  oxidizing 
them  to  ferric  with  an  oxidizing  agent  of  known  power,  the 
strength  of  the  ferrous  salt  being  determined  by  the  quantity 
of  the  oxidizing  agent  required  to  convert  it  to  ferric. 

Saccharated  Ferrous  Carbonate  (FeC03  =  115.82).  One 
gm.  of  saccharated  ferrous  carbonate  is  dissolved  in  10  mils  of 
diluted  sulphuric  acid  and  the  solution  diluted  with  water 
to  about  100  mils.  The  decinormal  potassium  dichromate  is 
carefully  added,  until  a  drop  of  the  solution  taken  out  and 
brought  in  contact  with  a  drop  of  freshly  prepared  solution 
of  potassium  ferricyanid  ceases  to  give  a  blue  color. 

The  number  of  mils  of  the  dichromate  solution  is  read  off 
and  the  following  equations  applied: 

6FeC03  +  6H2SO4  =  6FeS04  +  6H2O  +  6CO2 ; 

694.9  911-34 

then 

6FeC03    or    6FeS04  +  K2Cr207  +  7H2S04 

6)694.9  6)911.34        6)294.2 

10)115.84  10)151.89      10)49.03  jg. 

11.584  gms.  15.189  gms.     4.903  gms.,  or  1000  mils  —  K2Cr207  V.S. 

=  K2S04  +  Cr2(S04)3  +  7H20+3Fe2(S04)3. 

N 
Thus  each  mil  of  —  K2Cr207  represents  0.011584  gm.  of 

pure  ferrous  carbonate  or  0.005582  gm.  of  metallic  iron. 


ANALYSIS  BY  OXIDATION  AND  REDUCTION        185 

If  strong  sulphuric  acid  is  added  to  saccharated  ferrous 
carbonate  it  will  char  the  sugar,  and  a  black  mass  of  burnt 
sugar  is  obtained.  This  may  be  prevented  by  adding  water 
first,  and  then,  slowly,  the  sulphuric  acid. 

Instead  of  sulphuric  acid,  hydrochloric  acid  may  be  used. 


Fig.  45. 


This  will  not  char  the  sugar,  but  the  ferrous  chlorid  which 
is  then  formed  is  too  readily  oxidized  by  the  air. 

It  has  also  been  suggested  that  as  hydrochloric  acid  so 
rapidly  converts  ordinar}^  sugar  into  invert  sugar  as  to  render 
it  easily  attacked  by  the  dichromate,  it  should  be  cautiously 
used,  if  at  all.  Phosphoric  acid  has  none  of  these  disad- 
vantages, and  may  be  employed  with  good  results. 

In  making  estimations  of  ferrous  salts  with  potassium 
dichromate,  care  should  be  taken  to  avoid  atmospheric  oxida- 
tion. It  is  good  practice  to  calculate  approximately  how 
much  of  the  standard  solution  will  probably  be  required  to 


186      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

complete  the  oxidation,  and  then  add  almost  enough  of  the 
standard  solution  at  once,  instead  of  adding  it  slowly. 

A  white  porcelain  slab  is  then  got  ready,  and  placed 
alongside  of  the  flask  in  which  the  titration  is  to  be  performed. 
Upon  this  slab  are  placed  a  number  of  drops  of  the  freshly 
prepared  solution  of  potassium  ferricyanid,  and  at  intervals 
during  the  titration  a  drop  is  taken  from  the  flask  on  a  glass 
rod  and  brought  in  contact  with  one  of  the  drops  on  the 
slab.  The  glass  rod  should  always  be  dipped  in  clean  water 
after  having  been  brought  in  contact  with  a  drop  of  the 
indicator.     See  Fig.  45. 

When  a  drop  of  the  solution  ceases  to  give  a  blue  color 
on  contact  with  the  indicator,  the  reaction  is  complete. 

Pills  of  ferrous  carbonate  and  mass  of  ferrous  carbonate 
are  assayed  by  the  same  process. 

Ferrous  Sulphate  (FeSOi  +  7H2O  =  277.89).  Dissolve  about 
one  gram  of  crystallized  ferrous  sulphate  in  a  little  water, 
add  a  good  excess  of  .sulphuric  or  hydrochloric  acid,  titrate 
with  the  decinormal  potassium  dichromate,  as  directed  under 
Ferrous  Carbonate,  and  apply  the  following  equation : 

6(FeS04.7H20)   +  KsCrsOy  +   7H2SO4 

6)1667.34 

10)277^  j^ 

27.789  gms.,  or  1000  mils  —  K2Cr207  V.S. 

=  3Fe2(S04)3  +  K2SO4  +  Cr2(S04)3  +  49H2O. 

N 
Thus  each  mil  of  the  —  K2Cr207  V.S.  represents  0.027789 

gm.  of  crystallized  ferrous  sulphate  or  0.015 189  anhydrous. 
If  I  gm.  of  the  salt  is  taken  and  dissolved  as  above,  it  should 
require  about  37  mils  of  the  standard  solution,  equivalent 
to  about  100  per  cent. 


ANALYSIS  BY  OXIDATION  AND   REDUCTION        187 


TABLE  OF  SUBSTANCES  WHICH  MAY  BE  ESTIMATED  BY 
MEANS  OF  POTASSIUM  PERMANGANATE  OR  POTASSIUM 
DICHROMATE. 


Name. 


Molecular 

Weight. 

CrOa 

100 

HPH2O2 

66.04 

HNO3 

63.01 

HNO2 

47.01 

H2C20,+  2H20 

126.05 

.      Ba02 

169-37 

CaCl2 

III. 01 

Ca(PH202)2 

170.17 

FeCIg 

162.20 

Fe(PH202)3 

250.94 

Fe2(SO,)3 

399-85 

FeCOa 

115.82 

FeO 

71.82 

FeSO, 

151.89 

FeSO,+  7H20 

277.89 

Fe2 

55-82 

H2O2 

34-0. 

MnOj 

86.93 

KPH2O2 

104.15 

NaPH202 

88.05 

NaN02 

69.01 

N 


Factor. 


Acid,  chromic  

"     hypophosphorous 

"     nitric 

"     nitrous 

"     oxalic  (crystallized) 

Barium  dioxid 

Calcium  chlorid 

"         hypophosphite 

Ferric  chlorid ' 

"       hypophosphite 

"       sulphate 

Ferrous  carbonate 

"        oxid 

"        sulphate  (anhydrous) . 
"  "         (crystallized) 

Ferrum  (metallic) 

Hydrogen  dioxid 

Manganese  dioxid 

Pota.ssium  hypophosphite  .  .  .  . 

Sodium  hypophosphite 

"        nitrite 


0.0033 
0.00165 1 
0.0021 
0.00235 
o . 0063 
o . 00845 

0-00555 

0.002127. 

0.01622 

0.025094 

0.01999 

0.011582 

0.007182 

0.015189 

0.027789 

0.005582 

0.0017 

0.004346 

0.0026 

0.0022 

0.00345 


Analysis  by  Indirect  Oxidation 

This  method  of  analysis  is  based  upon  the  oxidizing  power 
of  iodin. 

lodin  acts  upon  the  elements  of  water,  forming  hydri- 
odic  ^cid  with  the  hydrogen,  and  liberating  oxygen  in  a  nascent 
state. 

Nascent  oxygen  is  au^|^ctive  agent,  and  readily  com- 
bines with  and  oxidize^^^^^Kubstances,  such  as  arsenous 
oxid,  sulphurous  acid,  si^JJj^thiosulphates,  hydrosulphuric 
acid,  the  lower  oxids  of  antimony  and  their  salts. 


188      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 
AS2O3  +  2H2O  +  2I2  =  As205+4Hi ; 

H2S03  +  H20  +  l2=2HI  +  H2S04. 

Therefore  iodin  is  said  to  be  an  indirect  oxidizer,  and 
may  be  used  for  the  estimation  of  a  great  variety  of  substances 
with  extreme  accuracy. 

When  iodin  is  brought  in  contact  with  certain  oxidizable 
substances  it  is  decolorized.  This  decolorization  occurs  as 
long  as  some  of  the  oxidizable  substance  is  present,  and  ceases 
when  oxidation  is  complete.  Hence  when  the  yellow  color 
of  iodin  shows  itself  in  the  solution  being  analyzed  the  reaction 
is  known  to  be  at  an  end.  In  most  cases  a  more  delicate 
end-reaction  is  obtained  by  using  starch  solution  as  an  indicator. 
This  gives  a  distinct  and  unmistakable  blue  color  with  the 
slightest  excess  of  iodin. 

In  making  an  analysis  with  standard  iodin  solution,  the 

substance  under  examination  is  brought  into  dilute  solution 

(usually  alkaline),  the  starch  solution  added,  and  then  the 

iodin,  in  the  form  of  a  standard  solution,  is  delivered  in  from 

a  burette,  stirring   or  shaking  constantly,  until  a  final   drop 

colors  the  solution  blue — a  sign  that  a  slight  excess  of  iodin 

has  been  added. 

N 
Preparation  of  Decinormal  Iodin   (1=126.92;    —  V.S.= 

12.692  gms.  per  liter).     Dissolve  12.692  gms.  of  pure*  iodin 

*  If  pure  iodin  be  not  at  hand,  it  may  be  prepared  from  the  commercial 
article  as  follows: 

Powder  the  iodin  and  heat  in  it  a  porcelain  dish  placed  over  a  water- 
bath,  stirring  constantly  with  a  glass  rod  for  twenty  minutes.  Any  adhering 
moisture,  together  with  any  cyanogen  iodid,  and  most  of  the  iodin  bromid 
and  iodin  chlorid,  is  thus  vaporized. 

Then  triturate  the  iodin  with  about  5  per  cent  of  its  weight  of  pure,  dry  potas- 
sium iodid.  The  iodin  bromid  and  chlorid  are  thereby  decomposed,  potassium 
bromid  and  chlorid  being  formed  and  iodin  liberated  from  the  potassium  iodid. 

The  mixture  is  then  returned  to  the  porcelain  dish,  covered  with  a  clean  glass 
funnel,  and  heated  on  a  sand-bath.     A  pure  resublimed  iodin  is  then  obtained. 


ANALYSIS  BY  OXIDATION  AND  REDUCTION       189 

in  300  mils  of  distilled  water  containing  18  gms.  of  pure  potas- 
sium iodid.  Then  add  enough  water  to  make  the  solution 
measure  at  25°  C.  exactly  1000  mils. 

The  solution  should  be  kept  in  small  glass-stoppered  vials, 
in  a  dark  place. 

The  potassium  iodid  used  in  this  solution  acts  merely  as 
a  solvent  for  the  iodin. 

If  pure  iodin  is  used  in  making  this  solution,  there  is  no 
necessity  for  checking  (standardizing)  it. 

But  if  desired  the  solution  may  be  checked  against  pure 
arsenous  acid  or  sodium  thiosulphate.  It  there  is  any  doubt 
as  to  the  purity  of  the  iodin,  it  is  best  to  take  a  larger  quantity, 
say  14  gms.  instead  of  the  12.692  gms.  directed  above,  and 
then  dilute  the  resulting  solution  to  the  proper  strength  after 
standardizing. 

Standardization  of  Iodin  V.S.  by  Means  of  a  Decinor- 
tnal  Sodiutn  Thiosulphate  Solution.  25  mils  of  the  iodin 
solution  are  accurately  measured  off  into  a  beaker,  and  then 

N  .  • 
from  a  burette  the  —  thiosulphate  is  delivered  until  the  solu- 
tion is  of  a  pale  yellow  color, -two  or  three  drops  of  starch  solu- 
tion are  then  added,  and  the  titration  with  the  thiosulphate 
solution  continued  until  the  blue  color  of  starch  iodid  is  dis- 
charged. 

If  the  iodin  solution  is  exactly  decinormal,  the  25  mils  will 
require  25  mils  of  decinormal  sodium  thiosulphate  to  exactly 
complete  the  reaction. 

If  on  the  other  hand  more  than  25  mils  of  thiosulphate 
solution  is  required,  it  indicates  that  the  iodin  solution  is  too 
concentrated,  and  must  be  diluted  so  as  to  correspond  with 
the  thiosulphate  solution,  volume  for  volume. 

Example.  Assuming  that  in  the  above  titration,  27  mils  of 
the  thiosulphate  solution  were  used,  then  each  25  mils  of  the 


190     THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

iodin  solution  must  be  diluted  with  water  to  make  27  mils  in 

order  to  convert  the  iodin  solution  into  a  strictly  decinormal 

solution.    If,  however,  the  iodin  solution  is  found  to  be  weaker, 

N 
as  evidenced  by  its  using  up  less  than  its  own  volume  of  — 

thiosulphate,  its  relative  strength  should  be  noted  on  the  label 

of  the  container. 

Thus  if  only  24.8  mils  of  the  thiosulphate  solution  are 

25 
used  up,  then  i  mil  of  the  latter  equals  — -  mils  or  1.008 

mils  of  the  iodin  solution. 

One  mil  of  this  iodin  solution  is  equivalent  to  0.992  mil 

N 
of  —  thiosulphate,  which  is  the  same  as  saying  i  mil =0.992 

N 
mil  of  —  iodin,  or  expressed  in  another  way,  i  mil  of  this 

iodin  solution  contains  0.01259+  gm.  of  iodin. 

Such  an  iodin  solution  may  be  used  as  an  empirical  solu- 
tion, and  in  any  assay  the  quantity  of  it  (in  mils)  which  is 

consumed  is  divided  by  1.008  or  multiplied  by  — -  or  by  0.992, 

25 
and  then  multiplied  by  the  decinormal  factor  for  the  sub- 
stance analyzed.  Another  way  is  to  multiply  the  mils  of  this 
iodin  solution  used  by  the  weight  of  iodin  contained  in  each 
mil,  and  then  by  a  fraction  in  which  the  numerator  represents 
the  quantity  of  the  substance  analyzed  equal  to  an  atom  of 
iodin,  and  the  denominator  is  the  atomic  weight  of  iodin. 

Example,    o.i  gm.  of  arsenous  acid  consumes  20  mils  of 
this  empirical  solution.  .How  much  absolute  AS2O3  does  it 

N 
contain?    The  —  factor  for  AS2O3  is  0.004948  gm. 

,^    7    7  ,  X     20X0.004948 

Method  (a) =  0.0981  gm. 


ANALYSIS  BY  OXIDATION  AND   REDUCTION       191 

24.8 
Method  (b)     20  X X  0.004948  =  0.0981  gm. 

Method  (c)     20X0.992X0.004948  =  0.0981  gm. 

49.48 
Method  (d)     20  X  0.0 12 59  X — - — =0.0981  gm. 

It  is  a  good  plan  to  have  the  factors  marked  on  the  labels. 
In  the  above  case  the  label  may  be  marked 

24.8  .,  .    ,. 

X or    X 0.992   or    i  mil    =0.01259  gm.  10dm. 

Standardization  of  lodin  V.S.  by  Means  of  Arsenoii 
Oxid.  0.2  gm.  of  pure  rcsublimed  vitreous  arsenous  oxid  is 
weighed  off  very  carefully  into  a  flask  50  mils  of  water  are 
added,  and  then,  after  the  addition  of  2  gms.  or  more  of 
sodium  bicarbonate,  the  mixture  is  gently  warmed  and  shaken 
until  the  arsenous  oxid  is  completely  dissolved.* 

To  this  solution  a  few  drops  of  starch  indicator  are  added, 
and  then  the  iodin  solution  delivered  carefully  from  a  burette 
until  a  blue  col^r  marks  the  end  of  the  reaction. 

AS2O3   +  I4  +   2H2O   =  AS2O5  +  4HI. 
197.92     4x126.92 

49.48  gms.  of  As203=  126.92  gms.  of  iodin; 
4.948   "     "  AS2O3--   12.692    "     "      "    or  1000  mils  —  V.S. 

*  Arsenous  oxid  is  much  more  readily  soluble  in  alkali  hydroxid,  than  in 
carbonated  alkalies,  therefore  the  following  method  of  making  the  solution 
is  preferred:  0.2  gm.  of  arsenous  oxid  is  dissolved  in  a  small  quantity  of 
boiling  water  with  the  aid  of  potassium  hydroxid  (free  from  sulphur),  the 
solution  is  then  acidified  with  hydrochloric  acid,  and  then  again  made  alkaline 
by  the  addition  of  sodium  bicarbonate.  The  latter  must  be  added  in  con- 
siderable excess,  being  careful,  however,  to  avoid  loss  of  solution  during 
Eervescence. 


192      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

0.2  gm.  of  AS2O3  will  require 

1000X0.2  '^      c     4.        ^  •  J-    TTc 

—  =40.44  mils  of  a  true  —  lodin  V.S. 

4.948        ^  10 

Assuming  that  in  the  above  titration  37.4  mils  of  the  iodin 
solution  were  used,  then  the  iodin  solution  is  too  concentrated 
and  must  be  diluted  so  that  each  37.4  mils  will  be  made  up  to 
40.44  mils. 

After  diluting  in  this  way  a  new  trial  should  be  made. 

It  is  a  good  plan  to  make  a  decinormal  solution  of  the 
arsenous  oxid  by  dissolving  4.948  gms.  of  the  pure  oxid  and 
30  gms.  of  sodium  bicarbonate  in  sufficient  water  to  m.ake 
laoo  mils  at  15°  C.  and  to  titrate  this  with  the  iodin  solution. 
25  mils  of  this  solution  should  require  for  complete  oxidation 
exactly  25  mils  of  the  iodin  solution,  if  the  latter  is  strictly  of 
decinormal  strength. 

The  Starch  Solution.  This  solution,  which  is  used  as  an 
indicator  in  iodometric  determinations,  is  made  as  follows : 

One  gm.  of  starch  (potato,  arrowroot,  or  corn  starch)  is  tri- 
turated with  10  mils  of  cold  water,  until  a  smooth  mixture  is 
obtained,  then  sufficient  boiling  water  is  added*  with  constant 
stirring,  to  make  200  mils  of  a  thin,  translucent  fluid.  If  the 
solution  is  not  translucent  it  should  be  boiled  for  about  three 
minutes,  then  allowed  to  cool,  and  filtered.  This  solution 
does  not  keep  very  long,  in  fact  it  becomes  useless  after 
standing  one  day,  therefore  it  should  be  freshly  prepared 
when  required. 

This  indicator  is  very  sensitive  to  iodin — it  will  detect 
one  part  of  iodin  in  3,500,000.  If  the  solution  is  not  clear, 
or  contains  flocks  of  insoluble  starch,  the  characteristic 
beautiful  blue  color  is  not  obtained  with  iodin;  instead,  a 
greenish  or  brownish  color  is  produced,   and   the  insoluble 


I 


ANALYSIS  BY  OXIDATION  AND  REDUCTION       193 

particles  are  even  colored  black  and  are  decolorized  with 
difficulty. 

The  blue  color  which  starch  gives  with  iodin  constitutes 
a  very  delicate  indication  of  the  slightest  excess  of  iodin. 
This  color  is  usually  regarded  as  being  due  to  the  formation 
of  a  compound  of  starch  and  iodin,  called  iodid  of  starch. 
It  is  a  compound  of  very  unstable  character  and  of  doubtful 
composition. 

Sodium  thiosulphate  behaves  towards  iodid  of  starch 
exactly  as  it  does  toward  free  iodin — it  takes  up  the  iodin 
and  thus  discharges  the  blue  color. 

Iodid  of  starch  dissociates  upon  heating,  but  reunites 
upon  cooling,  hence  it  is  advisable  to  avoid  heat  in  estimations 
where  starch  is  used  as  an  indicator. 

In  order  to  prevent  the  deterioration  of  this  solution  a 
few  drops  of  chloroform  may  be  added;  this  will  preserve 
it  for  a  long  time.  Oil  of  cassia  is  also  recommended  as  a 
preservative.  Moerk  adds  2  mils  of  the  oil  to  a  liter  of  the 
cooled  starch  solution.  Zinc  chlorid  or  iodid  added  to  the 
boiling  starch  solution  will  prevent  its  decomposition  for  a 
long  time.  A  starch  solution  so  made,  however,  should  not 
be  used  in  titrations  of  sulphids,  because  zinc  reacts  with 
sulphids. 

In  the  case  of  solutions  containing  carbonates,  the  pre- 
cipitate of  zinc  carbonate  is  so  small  in  amount  that  it  does 
not  interfere  in  the  least  with  the  recognition  of  the  end- 
reaction  tint.  Mercuric  iodid  is  also  a  very  valuable  preserva- 
tive. 

0.0 1  gm.  of  mercuric  iodid  in  a  liter  of  the  starch  solution 
is  quite  sufficient.  A  very  satisfactory  indicator  is  the  com- 
mercial soluble  starch  which  is  made  by  heating  potato  starch 
with  glycerin  and  precipitating  the  starch  by  repeated  treat- 
lent  with  alcohol.     This  starch  dissolves  readily  in  hot  water 


194      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

forming  a  clear  solution,  which  gives  a  very  delicate  reaction 
with  iodin.  It  is  best  preserved  under  alcohol,  the  latter 
being  removed  by  filtration  and  evaporation,  when  the  starch 
is  wanted  for  making  a  solution. 

In  making  starch  solution  for  use  as  an  indicator,  long 
continued  boiling  should  be  avoided,  as  this  converts  some 
of  the  starch  into  dextrin. 

On  the  Use  of  Sodium  Bicarbonate  in  Titrations  with 
Iodin.  In  these  titrations  an  excess  of  alkali  is  necessary  in 
order  to  neutralize  the  hydriodic  acid  formed. 

AS203  +  2H20  +  2I2  =  AS205  +  4HL 

If  the  hydriodic  acid  is  not  removed  by  neutralization  it 
will  react  with  the  arsenic  oxid  (AS2O 5), reducing  it  to  arsenous 
oxid  (AS2O3)  and  liberate  iodine,  as  shown  by  the  following 
equation : 

4HI  +  AS2O5  =  AS2O3  +  2H2O  +  2I2. 

Sodium  bicarbonate  is  usually  employed  to  neutralize  the 
HI  and  should  be  used  in  slight  excess. 

Alkali  hydroxids  or  carbonates  cannot  be  used  for  this 
purpose,  because  they  react  with  free  iodin  or  even  with 
starch  iodid.  Bicarbonates  ordinarily  have  no  such  action, 
and  therefore  sodium  bicarbonate  is  usually  directed  to  be 
added  in  excess  to  the  solution  to  be  titrated  with  iodin. 

It  is  well  known  that  sodium  hydroxid  solution  reacts 
with  free  iodin,  with  formation  of  hypoiodite  and  iodid. 

2NaOH+l2  =  NaIO+NaI+H20, 

the  hypoiodite  quickly  forming  iodate. 

3NaIO  =  2NaI  +  NaI03. 

It  is  also  now  a  recognized  fact  that  sodium  carbonate 


ANALYSIS  BY  OXIDATION  AND   REDUCTION       195 

is   partly   hydrolyzed   when   in    solution,    with   formation   of 
some  sodium  hydroxid,  as  per  equation, 

NaoCOa  +  H2O  =  NaOH  +  NaHCOs. 


the  hydroxid,  though  to  a  less  extent. 

On  the  other  hand,  it  is  generally  supposed  that  bicar- 
bonate of  soda  is  without  effect  on  iodin,  and  when,  in  iodo- 
metric  estimations,  addition  of  sodium  bicarbonate  is  indicated, 
little  attention  is  given  to  amount  added,  as  long  as  it  be  in 
excess. 

The  experiments  of  W.  A.  Puckner,  Proc.  A.  Ph.  A.,  1904, 
408,  prove  that  we  are  entirely  wrong  in  the  supposition  that 
sodium  "bicarbonate  has  no  effect  upon  iodin.  He  showed 
that  when  using  i  to  2  gms.  of  the  bicarbonate,  an  error  of 
1.5  to  4.5  mils  of  decinormal  iodin  may  be  introduced,  even 
when  the  sodium  bicarbonate  used  is  of  exceptional  purity, 
and  especially  proven  to  be  free  from  carbonate,  sulphite  or 
thiosulphate.  He  shows  that  when  sodium  bicarbonate  is 
added  to  a  decinormal  iodin  solution,  residual  titration  with 
sodium  thiosulphate  will  show  a  considerable  loss  of  free 
iodin,  which  went  into  combination  in  some  form  or  other 
(probably  iodid)  and  that  the  quantity  so  lost  is  proportional 
to  (i)  the  mass  of  sodium  bicarbonate;  (2)  the  time  of  the 
interaction  (the  reaction  is  slow);  (3)  the  concentration  of 
the  solution;  (4)  the  temperature,  and  (5)  the  size  of  the  flask 
in  which  reaction  occurs.  These  phenomena  are  due  to  the 
fact  that  sodium  bicarbonate  when  dissolved  in  water  under- 
goes hydrolysis,  thus 

2NaHC03  =  Na2C03+H2C03    or     (H2O  +  CO2). 

This  breaking  up  of  the  NaHCOs  into  Na2C03  and 
H2CO3,  and  the  latter  into  H2O  and  CO2,  continues  until  the 


196      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

pressure  of  the  CO2  above  is  equal  to  the  pressure  of  the  gas 
in  the  solution,  i.e.,  until  equilibrium  has  been  reached.  In 
concentrated  solutions  of  NaHCOa  the  amount  hydrolyzed  is 
much  greater  than  in  dilute  solutions.  An  elevation  of  temper- 
ature materially  increases  the  absorption  of  iodin. 

Less  iodin  is  lost  when  smaller  flasks  are  used,  provided 
the  glass  stopper  completely  shuts  off  communication  with 
the  atmosphere.  The  CO2  will  escape  from  the  solution 
until  its  pressure  in  the  solution  is  equal  to  that  of  the  gas 
above.  Thus,  since  a  larger  volume  of  air  is  contained  in 
a  larger  flask,  more  CO2  passes  from  the  liquid  before  equi- 
librium is  established,  hence  more  NaHCOs  is  decomposed, 
and  more  iodin  in  consequence  absorbed.* 

Reasoning  from  the  above  observations  it  may'  be  said 
that:  I,  though  sufficient  sodium  bicarbonate  be  used  to 
more  than  neutralize  the  hydriodic  acid  formed,  the  solution 
titrated  should  be  well  diluted;  2,  that  the  titration  should 
be  done  cold;  3,  that  the  titration  should  be  done  in  small 
stoppered  flasks,  and  4,  it  should  be  done  quickly. 

festimation  of  Arsenous  Compounds 

These  compounds  are  estimated  by  means  of  iodin  in  a 
manner  similar  to  that  described  under  standardization  of 
iodin  solution  by  means  of  arsenous  oxid.  The  method  is 
as  follows: 

Arsenous  Oxid  {Arsenous  Acid,  Arsenous  Anhydrid,  Arsenic 
Tr ioxid)  {AS2O s--^  J g J. g2).  When  arsenous  acid  is  brought  in 
contact  with  iodin  in  the  presence  of  water  and  an  alkali, 

*  For  further  study  of  equilibrium,  see  the  work  of  Dr.  H.  N.  McCoy, 
Am.  Ch.  J.,  vol.  XXIV,  437 


I 


ANALYSIS  BY  OXIDATION  AND  REDUCTION       197 

it  is  oxidized  into  arsenic  acid  and  the  iodin  is  decolorized. 
The  reaction  is: 

As203  +  2l2  +  2H20  =  As205  +  4HI;  % 

NaHCOa  +  HI  =  Nal + H2O  +  CO2. 

The  alkali  should  be  in  sufficient  quantity  to  combine 
with  the  hydriodic  acid  formed,  and  must  be  in  the  form  of 
potassium  or  sodium  bicarbonate. 

The  hydroxids  or  carbonates  should  not  be  used.  Starch 
solution  is  used  as  the  indicator,  a  blue  color  being  formed 
as  soon  as  the  arsenous  acid  is  entirely  oxidized  into  arsenic 
acid. 

Dissolve  about  0.2  gm.  of  arsenous  acid  accurately  weighed, 
in  20  mils  of  boiling  distilled  water  by  the  gradual  addition 
of  sodium  hydroxid  T.S.  until  complete  solution  results. 
Neutralize  this  solution  with  a  diluted  sulphuric  acid  V.S., 
using  phenolphthalein  T.S.  as  indicator,  cool,  dissolve  in  it 
2  gms.  of  sodium  bicarbonate  and  titrate  the  mixture  with 
decinormal  iodin  V.S.,  using  starch  T.S.  as  indicator,  shak- 
ing or  stirring  the  mixture  constantly  until  a  permanent  blue 
color  is  produced.  The  following  equation  illustrates  the 
reaction : 

AS2O3   +  2H2O   +   2I2  =  4HT   +  AS2O5. 

4)197-92  4)507-68. 

10)49.48  10)126.92 

4.948  gms.  12.692  gms.  or  1000  mils  —  I  V.S. 

N 
Thus  each  mil  of  —  I  V.S.  represents  0.004948  gm.  of 

pure  AS2O3. 

Solution   of  Arsenous   Acid   and   Solution    of    Potasfsium 
^»  Arsenite  are  assayed  in  the  manner  above  described.    Twenty 


198      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

added,  the  solution  diluted  to  loo  mils,  and  titrated  with  the 
decinormal  iodin  solution.  No  indicator  is  required  though 
stardj^may  be  used. 

In  the  case  of  solution  of  potassium  arsenite,  it  is  advisable 
to  slightly  acidify  with  hydrochloric  acid,  then  to  make  the 
solution  alkaline  with  sodium  bicarbonate  before  titrating. 
The  hydrochloric  acid  is  employed  here  in  order  to  neutralize 
any  potassium  hydroxid  which  may  have  been  formed  through 
hydrolysis  of  the  potassium  bicarbonate  contained  in  the 
solution. 

Arsencms  lodid  (Asl3  =  455.72).  This  salt  is  estimated  in 
the  same  way  as  described  for  arsenous  oxid.  The  reaction 
is  illustrated  by  the  following  equation: 

2ASI3  +  5H2O   +  2I2  =.As205  +   loHL 

4)911-44 

10)227-86  j^ 

■    22.786  gms.=  1000  mils  —  I  V.S. 


The  Direct  Percentage  Assay  of  Arsenous  Compounds. 
A  quantity  of  arsenous  acid  is  taken,  which  is  equal  to  the 
weight  of  pure  AS2O3,  oxidized  by  100  mils  of  decinormal 
iodin,  i.e.,  0.4948  gm. 

N 
If  0.4948  gm.  of  the  sample  be  taken  then  each  mu  of  — 

I  V.S.  will  represent  ywo  oi  this  quantity  or  i  per  cent  of 
pure  AS2O3.  In  the  case  of  weak  solutions  of  arsenic,  as 
liquor  acidi  arsenosi,  liquor  potassii  arsenitis,  etc.,  which 
contain  only  one  per  cent  of  arsenous  acid.  A  much  larger 
quantity  should  be  taken  for  analysis,  otherwise  the  quantity 
of  standard  iodin  solution  used  will  be  so  small  as  to  diminish 
the  accuracy  of  the  test. 

Thus,  if  only  0.4948  gm.  of  either  of  the  above  solutions 


ANALYSIS  BY  OXIDATION  AND   REDUCTION        199 

be  taken,  no  more  than  i  mil  of  the  standard  solution  would 
be  required.  It  is  better  to  take  enough  of  the  preparation 
to  use  up  30  to  50  mils  of  standard  solution. 

Estimation  of  Antimony  Compounds 

Antimonous  oxid  (Sb203)  or  any  of  its  compounds  may 
be  accurately  estimated  by  means  of  iodin,  in  a  manner  similar 
to  that  described  for  the  estimation  of  arsenous  oxid,  the 
antimonous  oxid  being  oxidized  to  antimonic  oxid,  as  per 
equation, 

SbgOs  +  2H2O  +  2I2  =  4HI  +  Sb205. 

The  antimonous  oxid  is  dissolved  and  kept  in  solution  by 

the  aid  of   tartaric   acid,  and  then  after  the  addition  of  an 

N 
excess  of  sodium-bicarbonate,  the  solution  is  titrated  with  — 

10 

iodin,  using  starch  as  an  indicator.  Accurate  results  can  only 
be  obtained  if  the  solution  is  sufficiently  alkaline  to  neutralize 
the  hydriodic  acid  formed  during  the  reaction.  The  titration 
should  be  conducted  without  delay  after  the  addition  of  the 
bicarbonate,  otherwise  a  precipitate  of  antimonous  hydrate 
will  be  formed,  upon  which  iodin  has  little  effect.  The  anti- 
mony must  be  in  solution  to  be  properly  attacked  by  the 
iodin. 

To  0.1  gm.  of  antimonous  oxid  20  mils  of  water  are  added 
and  the  mixture  heated  to  boiling;  to  this  tartaric  acid  is 
added  in  small  portions  at  a  time  until  the  oxid  is  completely 
dissolved.  The  solution  is  then  neutralized  by  means  of 
sodium  carbonate,  and  sufficient  of  a  saturated  solution  of 
sodium  bicarbonate  is  added  to  make  the  solution  distinctly 
alkaline  (about  10  mils[is  required  for  o.i  gm.  of  the  antimonous 
oxid).     The  mixture  is  now  ready  for  titration  with  standard 


200      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

iodin  solution.  This  should  be  done  immediately.  The 
appearance  of  a  permanent  blue  color  marks  the  end-point, 
starch  being  used  as  indicator. 

SbaOs   +  2H2O   +  2I2  =  4HI   +  SbaOs. 

4)288.4  4)507-68 

10)72.1  10)126.92  , 

7.21  gms.  12.692  gms.  or  1000  mils  — -*  V.S. 

10 

N 
One  mil  of  —  iodin  represents  0.00721  gm.  of  Sb203. 

The  solution  of  the  oxid  may  be  made  by  means  of  hydro- 
chloric acid,  and  after  adding  a  portion  of  tartaric  and  diluting 
with  water,  sodium  bicarbonate  is  added  and  the  titration 
conducted  as  above. 

Other  compounds  of  antimony  may  be  estimated  in  the 
same  way.  Antimonic  compounds  are  reduced  to  antimonous 
sulphid  (Sb2S3)  by  precipitating  with  hydrogen  sulphid,  and 
after  thoroughly  washing  the  precipitate,  dissolving  it  in  hydro- 
chloric acid;  thus  a  solution  of  antimonous  chlorid  is  obtained 
from  which  all  traces  of  hydrogen  sulphid  are  expelled  by 
boiling.  This  solution  is  diluted  with  water,  tartaric  acid 
added,  and  finally,  after  making  alkaline  with  sodium  bicar- 
bonate, titrated  with  the  standard  iodin  solution  as  above 
described. 

Antimony     and     Potassium     Tartrate    (Tartar    Emetic) 

[2(K[SbO]C4H406)+H20  =  664.7].     One   gm.    of   the   salt   is 

dissolved    in   sufficient   water    to    make    100   mils.      30    mils 

of  this  solution,   representing  0.3   gm.  of   the   salt,  are  taken 

for   assay.     20  mils  of  a  cold   saturated  solution  of  sodium 

bicarbonate   are   added,  then  a  little  starch  solution,  and  the 

N   .     . 
mixture  titrated   with  —  iodin  until  a  permanent  blue  colo.r 

appears. 

The  calculation  is  as  follows: 


ANALYSIS  BY  OXIDATION  AND   REDUCTION       201 

2K(SbO)C4H406   +  H2O   +   2I2   +  3H2O 

4)664.7  4)507-68 

10)166.17  10)126.92 

16.617  gms.  12.692  gms.  =  1000  mils  —  I  V.S. 

10 

=  4HI  +  2KHC4H4O6  +  2HSb03. 

N 
I  mil  of  —  iodin  represents  0.016617  gm.  of  K(SbO)C4H406 

+IH2O  (crystallized  tartar  emetic). 

K(SbO)C4H406  (anhydrous  tartar  emetic)  =323.34. 

2)323-34 
10)161.67  -j^ 

16.167  gnis.  =  1000  mils  —  V.S, 

N  . 
Thus  I  mil  of  —  iodin  represents  0.016167  gm.  of  anhydrous 

tartar  emetic. 

Estimation  of  Sulphurous  Acid  and  Sulphites 

These  substances  may  be  accurately  estimated  by  means 
of  a  standard  solution  of  iodin.  When  sulphurous  acid  or 
one  of  its  salts  is  brought  in  contact  with  iodin,  a  complete 
oxidation  takes  place.  The  sulphurous  acid  is  oxidized  to 
sulphuric  acid  and  the  sulphite  to  a  sulphate,  as  the  equations 
show : 

H2SO3  +  H2O  +12  =  2HI  +  H2SO4, 

NaaSOa  +  H2O  + 12  ==  2HI  +  Na2S04, 

NaHSOa  +  H2O  + 12  =  2HI  +  NaHS04. 

There  are  two  methods  which  may  be  employed.  In  one 
method  the  substance  is  brought  into  solution  in  water,  an 
excess  of  sodium  bicarbonate  is  added,  and  then  the  standard 
iodin  solution  is  rim  in  until  a  faint  yellow  color  of  free  iodin 


I 


202      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

marks  the  end-reaction.     If  starch  sohition  is  used  as  indicator 

the  end-point  is  the  production  of  a  blue  color.     The  other 

method  is  that  of  Giles  and  Shearer,  who,  in  a  ver}^  voluable 

series  of  experiments  detailed  in  the  J.  S.  C.  L,  III,  197,  and 

IV,  303,  suggest  the  following  modification: 

The   weighed   sulphurous   acid    or   the   sulphite    (in   fine 

N 
powder)   is  added  to  an  accurately  measured   excess  of  — 

iodin,   without  diluting  with   water.     After  the  mixture  has 

been   allowed   to   stand   for  about   one  hour,   with   frequent 

shaking,  the  oxidation  is  complete,  and  the  excess  of  iodin 

N 
is  ascertained  by  titrating  back  with  —  sodium  thiosulphate. 

The   quantity  of  the  latter  deducted   from   the   quantity 

N 
of  —  iodin  solution  added,  will  give  the  quantity  of  the  latter 

which  reacted  with  the  sulphite. 

The  neutral  and  acid  sulphites  of  the  alkalies,  alkali  earths, 
and  even  zinc  and  aluminum,  may  be  accurately  estimated 
in  this  manner.  The  less  soluble  salts  requiring,  of  course, 
more  time  and  shaking,  to  insure  their  complete  oxidation. 
The  latter  is  the  U.  S.  P.  method. 

Sulphurous  Acid.  This  is  an  aqueous  solution  of  sulphur 
dioxid  (802  =  64.07). 

Sulphurous  acid  when  brought  in  contact  with  iodin  is 
oxidized  into  sulphuric,  the  iodin  being  decolorized  because  of 
its  union  with  the  hydrogen  of  the  accompanying  water, 
forming  hydriodic  acid. 

Two  grams  of  sulphurous  acid  are  taken  and  diluted  with 
distilled  water   (recently  boiled    and    cooled  *)   to  about   25 

*  "Recently  boiled"  insures  absence  of  air,  the  oxygen  of  which  would 
partially  oxidize  the  sulphurous  acid,  and  "cooled"  is  directed  to  avoid  loss 
of  SO2,  which  would  occur  if  hot  water  were  used. 


ANALYSIS  BY  OXIDATION  AND   REDUCTION      203 

mils.  Two  grams  of  sodium  bicarbonate  are  added,  and  then 
the  decinormal  iodin  V.S.  is  delivered  into  the  solution  (to 
which  a  little  starch  solution  had  been  previously  added) 
until  a  permanent  blue  color  is  produced. 

The  following  equations,  etc.,  show  the  reactions  that  take 
place: 

H2S03+H20+l2-^2HT+H2S04. 

Sulphurous  acid  being,  however,  looked  upon  as  a  solu- 
tion of  SO2  in  water,  the  quantity  of  this  gas  is  generally 
estimated  in  analyses. 

H20,S02  +  H2O  +  I2  =  2HI  +  H2SO4. 

2)64.07  2)253.84 

10)32.04  10)126.92 

3.204  gms.  12.692  gms. 

N 
Thus  each  mil  of  —  iodin  consumed  before  the  blue  coior 

10 

appears,  represents  0.003204  gm.  of  SO2. 

The  Residual  Method.  Because  of  the  volatile  nature  of 
this  acid  the  residual  method  described  below  is  the  most 
satisfactory  in  that  loss  by  volatilization  is  avoided  and  com- 
plete oxidation  of  the  acid  assured.  When  the  direct  method 
described  is  used  there  is  more  or  less  loss  of  SO2  and  incom- 
plete oxidation,  with  separation  of  sulphur. 

Measure  2  mils  of  the  sulphurous  acid  into  a  stoppered 

weighing  flask  and  find  its  exact  weight.     Add  this  to  the 

N  .     .  .        .  ... 

;o  mils  of  —  iodin  contained  in  a  titration  flask  and  let  the 
10 

N 
solution  stand  for  about  five, minutes.     Then  titrate  with  — 

10 

sodium  thiosulphate  until  the  mixture  is  decolorized.      Sub- 
tract   the    number  of  mils  of  the  thiosulphate  used  from  the 


204      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 


N 
50  mils  of  —  iodin  added,  and  multiply  the  difference  by  the 

N 


10 


factor  for  SO2,  which  is  0.0032035  gm.     This  will  give  the 

weight  of  SO 2  in  the  quantity  of  acid  taken  for  analysis. 

When  a  solution  containing  sulphur  dioxid  is  to  be  measured 

by  means  of  a  pipette,  it  is  never  advisable  to  fill  the  instrument 

by  suction  in  the  usual  manner,  as  this  would 

cause  a  loss  of  the  gas.    A  better  plan  is  to  fill 

the   pipette   by   pressure   by   the   use   of   an 

arrangement  similar  to  that  shown  in  Fig.  46. 

The  solution  containing  sulphur  dioxid  or 

other  volatile  substance  is  poured  into  a  flask 

which  is  provided  with    a    stopper    through 

which   two   glass   tubes    pass;     one   of   these 

tubes  reaches  nearly  to  the  bottom  of  the  flask 

and  the  other  projects  about  one-half  an  inch 

below  the  stopper  and  is  bent  outward  above. 

To  the  upper  end  of  the  former  the  pipette  is 

attached  by  means  of  a  piece  of  rubber  tubing. 

By  blowing  into  the  flask  through  the  shorter 

tube  the  liquid  is  caused  to  rise  and  fill  the 

pipette,  which  may  then  be  easily  pulled  out  of 

the  rubber  tube  connection. 

Sodium  Sulphite  (Na2S03  +  7H2O  =  252.2) . 

Take  0.5  gm.  of  the  finely  powdered  crystals, 

N 
add  to  Ko  mils  of  —  iodin,  contained    in  a 
^  10 

loo-mil    glass-stoppered    flask,  and  allow  to  stand  for    one 

N 
hour  (shaking  frequently);   then- titrate  with  —   sodium   thio- 

sulphate  until  the  color  is  discharged. 
The  reaction  is  expressed  as  follows : 


Fig.  46. 


10 


ANALYSIS  BY  OXIDATION  AND   REDUCTION       205 

NaaSOs   +  7H2O   +  h  =   2HI  +  Na2S04  +  6H2O. 

2)252.2  2)253.84 

10)126.1  10)126.92  -jyj 

12.61  gms.  12.692  gms.  or  1000  mils  —  iodin  V.S. 

Thus  each  mil  of  the  standard  solution  represents  0.01261 
gm.  of  crystallized  sodium  sulphite. 

If  I  gm.  of  the  salt  is  taken,  to  find  the  percentage  multi- 
ply the  factor  by  the  number  of  mils  of  standard  solution 
consumed,  and  the  result  by  100. 

Potassium  Sulphite  (K2SO3  + 21120  =  194.37).  Operate 
upon  0.5  gm.  in  the  same  manner  as  for  sodium  sulphite. 

K2SO3   +   2H2O   +  I2  =   2HI   +  K2SO4  +  H2O. 

2)194-37 
10)  97.18 

9.718  gms.  or  1000  mils  of  standard  V.S. 

N 

Each  mil  of  the  —  iodin  represents  0.009718  gm.  of  crys- 
tallized potassium  sulphite. 

Sodium  Bisulphite  (NaHS04=  104.08).  Operate  upon 
about  0.25  gm.  in  the  same  manner  as  for  sodium  sulphite, 
and  apply  the  following  equation: 

NaHS03  +  l2  +  H20  =  2HI  +  NaHS04. 

Sodium  Thiosulphate  (Sodium  Hyposulphite)  (Na2S203 
+  51120  =  248.24).  This  salt,  when  brought  in  contact  with 
iodin,  is  converted  into  sodium  iodid  and  sodium  tetrathionate. 
The  reaction  is  expressed  by  the  equation 

2Na2S203  + 12  =  2NaI  +  NaaSjOe. 
It  is  estimated  as  follows:    i  gm.  of  the  salt  is  dissolved 


206     'THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

in  20  mils  of  water,  a  few  drops  of  starch  solution  are  added, 

N 
and  then  the  —  iodin  is  delivered  in  from  a  burette,  until 
10  ' 

the  appearance  of  blue  starch  iodid  indicates  an  excess  of 

iodin. 

Hydrogen  Sulphid  (H2S  =  34.07) .     When  iodin  and  hydrogen 

sulphid  are  brought  together  in  solution  the  following  reaction 

occurs: 

H2S  +  2l  =  2HI  +  S. 

The  reaction  is  not  regular,  howeverj  when  performed  in 
an  acid  solution,  but  in  the  presence  of  alkali  bicarbonates 
the  results  are  constant.  The  method  may  be  employed  for 
the  estimation  of  alkali  sulphates. 

The  process  may  be  conducted  as  follows: 

Into  30  mils  of  a  cold  saturated  solution  of  sodium  bi- 
carbonate, contained  in  a  500-mil  flask,  measure  a  suitable 
quantity  of  the  solution  of  hydrogen  sulphid,  stopper  the 
flask  and  mix  contents  by  shaking.  Dilute  the  solution 
with  about  300  mils  of  water,  add  starch  solution  and  titrate 

N 
with  —  iodin  V.S.  until  a  distinct  and  permanent  blue  color 

appears. 

N 
Each  mil  of  —  iodin  represents  0.0017035  gm.  of  H2S. 

The  residual  method  may  also  be  employed.     A  suitable 

N  . 
volume  of  the  sample  is  added  to  an  excess  of  —  iodin  V.S. 
^  10 

mixed  with  some  sodium  bicarbonate  solution,  the  solution  is 

N 
thoroughly  shaken,  and  then  titrated  with  —  thiosulphate ;  the 

N 
quantity  of  the  latter,  deducted  from  the  quantity  of  —  iodin 


ANALYSIS  BY  OXIDATION  AND   REDUCTION 


207 


—  iodin 

lO 


which   reacted   v/ith 


added,    gives    the   quantity   of 

the  H2S. 

Sulphids.  Soluble  sulphids 
may  be  estimated  by  either  of 
the  above  methods.  The  solu- 
tion of  sulphid  containing 
about  0.2  gm.  being  treated 
like  an  H2S  solution. 

Sulphids  insoluble  inwate? 
but  decomposable  by  dilute 
acids  may  be  estimated  as  fol- 
lows : 

A  weighed  quantity  of  the 
sulphid  is  introduced  into  a 
fiask,  provided  with  a  double 
perforated  stopper;  through 
one  of  the  perforations  the  stem  of  a  separatory  funnel  is 
passed,  through  the  other  a  glass  delivery  tube  (see  Fig.  47). 
The  funnel  tube  extends  nearly  to  the  bottom  of  the  flask  and 
is  bent  to  form  a  hook,  the  opening  of  which  is  under  water. 
The  delivery  tube  begins  at  the  lower  end  of  the  stopper  and 
ends  in  another  flask  containing  sodium  bicarbonate  solution. 
The  funnel  contains  diluted  sulphuric  acid,  which,  upon  open- 
ing the  glass  stop-cock,  is  allowed  to  flow  into  the  flask,  upon 
the  contained  sulphid;  the  H2S  liberated  is  conducted  into 
the  solution  of  sodium  bicarbonate  which  absorbs  it  com- 
pletely. A  current  of  air  aspirated  through  the  apparatus 
insures  absorption  of  the  entire  H2S  developed.  The  sodium 
bicarbonate  solution  of  H2S  is  then  titrated  with  the  standard 
iodin,  in  the  presence  of  starch. 


Fig. 


47- 


208      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

TABLE  OF  SUBSTANCES  WHICH  MAY  BE  ESTIMATED  BY  MEANS 
OF  STANDARD  lODIN  SOLUTION 


Name. 


Acid,  sulphurous 

Antimonous  oxid 

Antimony  and  potas'm  tartrate 

Arsenous  iodid 

"       oxid 

Cyanogen  

Hydrogen  sulphid 

Iron  (metallic) 

Mercuric  chlorid 

Mercurous  chlorid 

Potassium  cyanid 

"         sulphite  (anhydrous) 

"  (crystallized) 

Sodium  bisulphite 

"       sulphite  (anhydrous).. 

"  "        (crystallized) . 

"       thiosulphate 

Sulphur  dioxid 

Tin  in  stannous  compounds  .. . 
Zinc 


Formula. 


H2SO, 
SbjOg 

2[K(SbO)C,H606]  +  H20 

AST3 

AS2O3 

CN 

H2S 

Fej        ' 
HgClj 
HgCl 
KCN 
K2SO3 
K2SO3+2H2O 
NaHSOg 
Na2S03 
Na2S03+7H20 
Na2S203+5H20 
SO2 
Sn2 
Znj 


Molecular 
Weight. 


82.09 
288.4 
664.7 
455-72 
197.92 

26.01 

34  09 
III. 68 
271.52 
236.06 

65.11 
158.27 

194-30 
104.08 
126.07 
252.18 
248.22 
64.07 
238.0 
130 -74 


N 

— Factor. 


0.004103 

0.00721 

O.016617 

0.022786 

O . 004948 

0.0013005 

0.0017035 

0.002791 

0.027092 

0.023546 

0.003255 

0.007913 

0.009718 

o . 005  204 

0.006303 

O.OI26I 

0.024824 

0.0032035 

0.00595 

o .  003  268 


Estimation  of  Substances  Readily  Reduced. 

Any  substance  which  readily  yields  oxygen  in  a  definite 
quantity,  or  is  susceptible  of  an  equivalent  action,  which 
involves  its  reduction  to  a  lower  quantivalence,  may  be  quan- 
titatively tested  by  ascertaining  how  much  of  a  reducing  agent 
of  known  power  is  required  by  a  given  quantity  of  the  sub- 
stance for  its  complete  reduction. 

The  principal  reducing  agents  which  may  be  employed 
in  volumetric  analysis  are  sodium  thiosulphate,  sulphurous  acid, 
arsenous  acid,  oxalic  acid,  metallic  zinc,  and  magnesium. 

The  sodium  thiosulphate  is  the  only  one  which  is  employed 
officially  in  the  U.  S.  P.  in  the  form  of  a  volumetric  solution. 


ANALYSIS  BY  OXIDATION  AND  REDUCTION       209 

It  is  used  in  the  estimation  of  free  iodin,  and  indirectly  of 
other  free  halogens,  or  compounds  in  which  the  halogen  is 
easily  liberated,  as  in  the  hypochlorites,  etc. 

Estimations  Involving  the  Use  of  Sodium  Thiosulphate  V.S. 

(lodometry) 

When  sodium  thiosulphate  acts  upon  iodin,  sodium  tetra- 
thionate  and  sodium  iodid  are  formed,  and  the  solution  is 
decolorized. 

This  reaction  takes  place  in  definite  proportions:  one 
.molecular  weight  of  the  thiosulphate  absorbs  onejitpmic  weight 
of  iodin. 

2Na2S203  + 12  =  2NaI  +  Na2S406. 

Chlorin  cannot  be  directly  titrated  with  the  thiosulphate, 
but  by  adding  to  the  solution  containing  free  chlorin  an  excess 
of  potassium  iodid,  the  iodin  is  liberated  in  exact  proportion 
to  the  quantity  of  chlorin  present,  atom  for  atom. 


I 


Cl2  +  2KI  =  2KCl+l2- 

Then  by  estimating  the  iodin,  the  quantity  of  chlorin  is 
certained.     All   bodies   which   contain   available   chlorin   or 
romin,  or  which  when  treated  with  an  acid  evolve  chlorin 
or  bromin,  may  be  estimated  by  this  method. 

Also,  bodies  which  contain  available  oxygen,  and  which 
when  boiled  with  hydrochloric  acid  evolve  chlorin,  such  as 
manganates,  chromates,  peroxids,  etc.,  may  be  estimated  in  this 
way. 

^Mt .   Solutions  of  ferric  salts,  when  acidulated  and  boiled  with 
^ftn  excess  of  potassium  iodid,  liberate  iodin  in  exact  propor- 


210      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

Arsenates,  copper  salts,  and  others  also  liberate  iodin  from 
'v^y    potassium  iodid,  quantitatively. 

'  ^  Thus  sodium  thiosulphate  may  be  used  in  the  estimation 

of  a  great  variety  of  substances  with  extreme  accuracy. 
\  Preparation  of  Decinormal  Sodium  Thiosulphate   (Hypo- 

Vsulphite)  (Na2S20H- 51120  =  248.22;  contains  24.822  gms.  in 
I  liter).  Sodium  thiosulphate  is  a  salt  of  thiosulphuric  acid 
in  which  two  atoms  of  hydrogen  have  been  replaced  by  sodium; 
it  therefore  seems  that  a  normal  solution  of  this  salt  should 
contain  one-half  the  molecular  weight  in  grams  in  one  liter. 

But  this  salt  is  used  chiefly  for  the  estimation  of  iodin, 
and,  as  stated  before,  one  full  molecular  weight  reacts  with 
and  decolorizes  one  atomic  weight  of  iodin,  and  since  one 
atom  of  iodin  is  chemically  equivalent  to  one  atom  of  hydrogen, 
a  full  molecular  weight  of  sodium  thiosulphate  must  be  con- 
tained in  a  liter  of  its  normal  solution. 

Sodium  thiosulphate  is  easily  obtained  in  a  pure  state, 
and  therefore  the  proper  weight  of  the  salt,  reduced  to  powder 
and  dried  between  sheets  of  blotting-paper,  may  be  dissolved 
directly  in  water,  and  made  up  to  one  liter. 

A  stronger  solution  than  decinormal  is  usually  made,  its 
titer  found,  and  then  the  solution  diluted  to  the  proper  measure. 

Thirty  gms.  of  selected  crystals  of  the  salt  are  dissolved  in 
enough  water  to  make,  at  or  near  25°  C,  1000  mils. 

This  concentrated  solution  is  then  standardized  by  one  of 
the  following  methods : 

a.  Standardization  hy  Means  of  -jq  Iodin,     Transfer  10 

mils  of  this  solution  into  a  flask  or  beaker,  add  a  few  drops  of 
starch  T.S.,  and  then  gradually  deliver  into  it  from  a  burette 
decinormal  iodin  solution,  in  small  portions  at  a  time,  shaking 
the  flask  after  each  addition,  and  regulating  the  flow  to  drops 
toward  the  end  of  the  operation.     As  soon  as  a  blue  color  is 


ANALYSIS  BY  OXIDATION  AND   REDUCTION       211 

produced  which  does  not  disappear  upon  shaking,  but  is  not 
deeper  than  pale  blue,  the  reaction  is  completed.  Note  the 
number  of  mils  of  iodin  solution  used,  and  then  dilute  the 
thiosulphate  solution  so  that  equal  volumes  of  it  and  the  deci- 
normal  iodin  will  exactly  correspond  to  each  other,  under  the 
above-mentioned  conditions. 

Example.  The  lo  mils  of  sodium  thiosulphate,  we  will 
assume,  require  10.7  mils  of  decinormal  iodin. 

The  sodium-thiosulphate  solution  must  then  be  diluted 
in  the  proportion  of  10  mils  to  10.7  mils,  or  1000  mils  to  1070 
mils. 

After  the  solution  is  thus  diluted  a  new  trial  should  be 
made,  in  the  manner  above  described,  in  which  50  mils  of 
the  thiosulphate  solution  should  require  exactly  50  mils  of 
the  decinormal  iodin  to  produce  a  faint  blue  color. 

The  solution  should  be  kept  in  small  dark  amber-colored, 
glass-stoppered  bottles,  carefully  protected  from  dust,  air, 
and  light. 

One  mil  of  this  solution  is  the  equivalent  of: 

Iodin 0.012692  gram. 

Bromin 0.007992       '' 

Chlorin 0.003546       '' 

•  Iron  in  ferric  salts 0.005584       " 

h.  Standardization  by  Means  of  Potassium  Dichrotnate, 

The  potassium  dichromate  should  be  pure,  if  not  it  should  be 
purified  by  triple  recrystallization  and  then  heated  in  a  porce- 
lain crucible  until  the  entire  mass  is  just  fused;  then  set 
aside  to  cool  in  a  desiccator  over  calcium  chlorid  or  sulphuric 
acid.  The  mass  falls  to  a  crystalline  powder.  Of  this  purified 
potassium  dichromate,  weigh  off  a  definite  quantity,  say  0.2 
gm.,  dissolve  it  in  a  small  quantity  of  water,  and  pour  the 
solution  into  a  beaker  containing  2  gms.  of  pure  potassium 


212      THE   ESSENTI/O^S   OF   VOLUMETRIC   ANALYSIS 

iodid  (free  from  iodate)  and  loo  mils  of  water.  Acidulate  the 
solution  with  5  mils  of  concentrated  hydrochloric  or  sulphuric 
acid,  cover  the  beaker,  and  let  stand  for  about  five  minutes, 
then  titrate  with  the  thiosulphate  solution  to  be  standardized 
(using  starch  as  an  indicator)  until  the  blue  color  is  just 
discharged.     The  calculation  is  then  made  as  follows: 

KaCrsOr  +  6KI  +  i4HCl  =  2CrCl3  +  8KC1  +  7H2O  +  3I2. 
294.4  996.12  761-52 

Thus  294.4  gms.  of  potassium  dichromate  oxidizes  996.12 
gms.  of  potassium  iodid  and  liberates  therefrom  761.52  gms. 
of  iodin. 

The  potassium  iodid  must  be  in  excess;  in  fact  for  each 
atom  of  iodin  liberated  one  molecule  of  potassium  iodid  must 
be  present  at  the  completion  of  the  reaction  in  order  to  keep 
the  iodin  in  solution,  and  thus  prevent  loss  by  volatilization. 

If  294.4  gms.  of  potassium  dichromate  liberate  761.52  gms. 
of  iodin,  0.2  gm.  will  liberate 

761.52X0.2  r    .      J. 

=  0.^173  gm.  of  10dm. 

294.4  0/0   6 

N 
0.012692  gm.  of  iodin  =  I  mil  of  —  thiosulphate. 

'N 
0-5173        "     "     *'     =40.172  mils  of — thiosulphate. 

Therefore  if  in  the  above  assay  37.60  mils  of  the  thiosul- 
phate V.S.  were  consumed,  it  must  be  diluted  so  that  each 
37.60  mils  will  measure  40.72  mils  in  order  to  convert  the  thio- 
sulphate solution  into  a  true  decinormal  solution.  A  new 
trial  should  then  be  made  with  the  diluted  solution  to  see  if 
its  strength  is  correct. 

It  is  usually  more  convenient  to  use  a  decinormal  dichro- 


I 


ANALYSIS  BY  OXIDATION  AND   REDUCTION     •  213 


mate  V.S.  for  standardizing  decinormal  thiosulphate  V.S. 
This  may  be  done  as  follows : 

Dissolve  two  grams  of  pure  potassium  iodid  in  a  small 
quantity  (lo  to  15  mils)  of  sulphuric  acid  (i-io).  Place  this 
into  a  500-mil  flask  and  add  to  it  very  slowly  25  mils  (accu- 
rately measured)  of  decinormal  potassium  dichromate  V.S., 
mix  well  and  let  stand  for  five  minutes,  the  flask  being  kept 
closed.  The  thiosulphate  solution  to  be  standardized  is  then 
run  in  from  a  burette  in  small  portions,  shaking  after  each 
addition  until  the  solution  is  a  pale  yellow  color.  Two  mils 
of  starch  solution  are  then  added  and  the  titration  continued 
drop  by  drop  until  the  blue  color  of  starch  iodid  is  just  dis- 
charged. 

Several  trials  are  made,  and  from  an  average  of  three  or 
four  closely  agreeing  results  the  quantity  of  water  required 
for  dilution  to  decinormal  strength  is  readily  calculated. 

Example.  Assuming  that  in  the  above  trial  23.8  mils  of 
the  thiosulphate  solution  were  required  to  react  with  the  iodin 
liberated  by  25  mils  of  decinormal  dichromate,  then  the  thio- 
sulphate* must  be  diluted  with  distilled  water  so  that  each 
23.8  mils  will  measure  25  mils.  After  dilution  a  new  trial 
should  be  made,  in  which  25  mils  of  decinormal  dichromate 
should  require  when  treated  as  above  described  exactly  25  mils 
of  the  thiosulphate. 

c.  StandardizatioTi  hy  Means  of  Potassium  Bi-iodate, 
This  method  depends  upon  the  fact  that  when  potassium  iodid 
and  bi-iodate  are  brought  together  in  the  presence  of  a  small 
quantity  of  an  acid,  an  equivalent  amount  of  iodin  is  set  free. 
The  reaction  is  illustrated  by  the  equation: 

KH(I03)2   +   loKI  +   iiHCl  =    12I   +    iiKCl   +  6H2O. 

12)389.94  12)1523.04 

io)32.49.S  10)126.92  j^ 

3.2495  gms,  12.692  gms.  or  1000  mils  —  V.S. 


214  •   THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

Thus  it  is  seen  that  one  molecule  of  the  bi-iodate  causes 
the  liberation  of  12  atoms  of  iodin,  against  which  the  thio- 
sulphate  solution  is  standardized. 

In  order  not  to  use  up  too  large  a  quantity  of  the  thio- 
sulphate  solution  in  the  titration,  a  very  small  quantity  of 
the  bi-iodate  must  be  taken,  and  since  a  small  error  in  weighing 
this  would  entail  a  relatively  large  error  in  the  results  it  is 
best  to  use  the  bi-iodate  in  the  form  of  a  solution  of  known 
strength  and  thus  obviate  the  difficulty. 

A  decinormal  solution  of  the  bi-iodate,  i.e.,  one  containing 
in  1000  mils  3.2245  gms.  of  the  salt  may  be  used  to  advantage. 
Such  a  solution  will  keep  unchanged  for  years,  and  may  be 
employed  as  follows  for  the  standardization  of  sodium  thio- 
sulphate  solution : 

Into  a  glass-stoppered  flask  of  250  mils  capacity  introduce 
10  mils  of  a  5  per  cent  solution  of  potassium  iodid,*  i  mil 
of  diluted  hydrochloric  acid  and  exactly  25  mils  of  the  above 
potassium  bi-iodate  solution. 

This  brownish-yellow  solution  is  now  titrated  with  the 
sodium  thiosulphate  solution  which  is  slowly  delivered 
from  a  burette  until  the  solution  becomes  pale  yellow  in 
color;  a  few  drops  of  starch  solution  are  now  added,  and 
the  titration  continued  (the  flow  being  reduced  to  drops) 
until  the  blue  color  is  just  discharged.  During  the  titra- 
tion, the  flask  should  be  frequently  stoppered  and  vigorously 
shaken. 

When  the  blue  color  is  discharged,  note  the  number  of 
mils  used.  If  an  exactly  decinormal  thiosulphate  solution  is 
taken,  25  mils  will  be  required  to  react  with  the  25  mils  of 
potassium  bi-iodate  solution,  under  the  above  conditions.     If, 

*  The  potassium  iodid  must  be  in  sufficient  quantity  not  only  to  react 
with  the  bi-iodate  quantitatively,  as  shown  in  the  equation,  but  also  to  dissolve 
the  iodin  which  is  liberated. 


L  ANALYSIS  BY  OXIDATION  AND   REDUCTION       215 

on  the  other  hand,  only  22  mils  of  the  thiosulphate  solution 
are  used  in  the  titration,  then  the  latter  is  too  strong,  and 
must  be  diluted  so  that  each  22  mils  will  measure  25  mils  in 
order  to  make  it  strictly  decinormal. 

d.  Standardization  by  Means  of  Potassium  Perrnanga- 
nate.  Sodium  thiosulphate  solution  may  be  accurately 
standardized  by /means  of  potassium  permanganate,  if  the 
other  substances  used  for  this  purpose  are  not  at  hand.  The 
method  is  easily  understood  by  referring  to  the  iodometric 
standardization  of  potassium  permanganate  solution.  See 
page  145- 

Estimation  of  Free  lodin  (1=126.92).  lodin  in  the  dry 
state  or  in  the  form  of  a  tincture  is  brought  into  aqueous 
solution  by  means  of  pure  potassium  iodid  and  then  titrated 
with  standard  sodium  thiosulphate.  The  potassium  iodid  is 
used  here  to  dissolve  the  iodin;  it  must  be  free  from  iodate 
(KIO3)  because  the  presence  of  this  salt  would  cause  a  libera- 
tion of  iodin  from  the  potassium  iodid. 

Dry  iodin  is  assayed  as  follows: 

About  0.5  gm.  of  iodin  are  placed  in  a  tightly  stoppered 
weighing  bottle  and  accurately  weighed.  One  gram  of  potas- 
sium iodid  and  50  mils  of  water  are  added,  and  when  the  iodin 

N 
is  dissolved,  —  sodium  thiosulphate  is  delivered  from  a  burette 

in  small  portions  at  a  time,  shaking  after  each  addition  until 
the  reaction  is  nearly  completed,  and  the  solution  is  of  a  faint 
yellow  color.  A  few  drops  of  starch  indicator  are  now  added, 
and  the  titration  with  the  thiosulphate  continued  drop  by 
drop  until  a  final  drop  just  discharges  the  blue  color.  The 
number  of  mils  of  the  thiosulphate  solution  used  is  noted. 
This  number,  multiplied  by  the  decinormal  factor  for  iodin, 
gives  the  weight  of  the  latter  present  in  the  sample 
assayed. 


216      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

I2  +  2(Na2S203  +  5H2O)  =  Na2S406  +  2NaI  +  10H2O. 

2)253.84  2)406.96 

10)126.92  10)248.48  js^ 

12.692  gms.  24.848  gms.  or  1000  mils  —  V.S. 

N 
Each  mil  of  —  thiosulphate  represents   0.012692  gm.   of 

iodin. 

N 
If  in  the  above  assay  39  mils  of  —  sodium  thiosulphate 

were  consumed,  then 

0.012692X39  =  0.495  gm. 

0.495  +  ^^^ 


0-5 


=  99  per  cent. 


LugoPs  Solution.  This  is  an  aqueous  solution  of  iodin 
and  potassium  iodid. 

It  is  estimated  for  iodin  in  the  same  way  as  the  foregoing. 
The  potassium  iodid  acts  merely  as  a  solvent  for  free  iodin, 
and  does  not  enter  into  the  reaction. 

Ten  or  twelve  grams  of  the  solution  is  a  convenient  quantity 
to  operate  upon.     Starch  solution  is  the  indicator. 

Tincture  of  Iodin.  An  alcoholic  solution  of  free  iodin 
must  be  diluted  with  a  solution  of  potassium  iodid,  before 
titration,  in  order  to  prevent  the  precipitation  of  iodin,  which 
would  result  upon  the  addition  of  the  aqueous  standard  solu- 
tion. The  U.S. P.  tincture  of  iodin  contains  potassium  iodid 
and  therefore  may  be  titrated  as  it  is. 

Indirect  lodometric  Estimations.  The  titration  methods 
previously  described,  in  which  iodin  is  used  as  a  standard 
solution  and  the  estimation  of  free  iodin  by  means  of  sodium 
thiosulphate,  are  classed  as  direct  iodometric  methods.  The 
following  methods,  in  which  the  strength  of  the  substance 
under  analysis  is  determined  by  the  quantity  of  iodin  which 


ANALYSIS  BY  OXIDATION  AND   REDUCTION       217 

it  liberates  from  an  iodid,  are  known  as  indirect  iodometric 
methods.  Potassium  iodid  is  added  in  excess  *  to  an  acidulated 
solution  of  the  substance,  and  the  liberated  iodin  estimated 
by  means  of  standard  thiosulphate.  These  methods  are 
among  the  most  accurate  of  all  volumetric  analyses,  and 
take  in  a  very  large  class  of  substances.  Among  the  sub- 
stances which  may  be  analyzed  by  this  method  are  chlorin 
and  bromin  and  all  substances  which  readily  liberate  these 
elements:  ferric  salts,  manganates,  chromates,  metallic  perox- 
ids,  and  other  substances  from  which  oxygen  can  be  easily 
liberated. 

Free  Chlorin  or  Bromin.  Free  chlorin  acts  upon  potas- 
sium iodid,  liberating  iodin,  as  per  the  equation 

Cl2  +  2KI=2KCl+l2. 

Thus  it  is  seen  that  each  atom  of  chlorin  will  liberate  one 
atom  of  iodin,  hence  by  determining  the  quantity  of  iodin 
by  means  of  a  standard  thiosulphate  solution  the  quantity 
of  chlorin  present  is  easily  ascertained  (126.92  gms.  of 
1  =  35.46  gms.  of  CI).  The  same  applies  to  free  bromin, 
one  atom  of  bromin  (79.92)  will  liberate  one  atom  of  iodin 

N 
(126.92)  Br2  +  2KI  =  2KBr  +  l2.    1000  mils  of  —  sodium  thio- 
sulphate is  equivalent  to  12.692  gms.  of  iodin,  and  hence  to 
3.546  gms.  of  chlorin  or  7.992  gms.  of  bromin. 

N      . 
Thus    I  mil  of  —  sodium   thiosulphate  is   equivalent  to 

0.012692  gm.  of  iodin;  0.003546  gm.  of  chlorin;  0.007992 
gm.  of  bromin. 

Chlorin  cannot  be  directly  titrated  with  sodium  thiosul- 

*  The  iodid  should  be  in  sufficient  excess  to  keep  the  liberated  iodin  in 
solution  as  KI.I. 


218      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

phate  because,  instead  of  the  tetrathionate  being  formed  as 
with  iodin,  sulphuric  acid  is  produced;  furthermore,  there  is 
no  readily  observable  end-point  as  there  is  with  iodin. 

Chlorin  Water.  This  is  an  aqueous  solution  of  chlorin, 
0  =  35.46,  containing  at  least  0.4  per  cent  of  the  gas. 

The  estimation  of  chlorin  is  effected  in  an  indirect  way, 
namely,  by  determining  the  quantity  of  iodin  which  it  liberates 
from  potassium  iodid. 

A  definite  quantity  of  chlorin  will  liberate  a  definite  quan- 
tity of  iodin  from  an  iodid;  these  quantities  are  in  exact 
proportion  to  their  atomic  weights,  as  the  equation  shows : 

CI2   +  2KI  =   2KCI   +  I2. 

2)70-92  2)253.84 

;o)35.46  10)126.92 


3.546  12.692  gms. 

Thus  it  IS  seen  that  by  estimating  the  liberated  iodin  the 
quantity  of  chlorin  may  be  determined  "with  accuracy. 

Ten  grams  is  a  convenient  quantity  to  operate  upon.  To 
this  about  half  a  gram  of  potassium  iodid  is  added.  A  little 
starch  solution  is  then  introduced,  and  the  titration  is  begun 
with  decinormal  sodium  thiosulphate. 

When  the  blue  color  of  starch  iodid  has  entirely  disappeared 
the  reaction  is  finished. 

The  reaction  between  iodin  and  sodium  thiosulphate  is 
illustrated  by  the  following  equation: 

I2  +  2(Na2S203  +  5H2O)  =  2NaI+  Na2S406  +  10H2O. 

2)253.84  2)496.96 

10)126.92  10)248.48  j^ 

12.692  gms.  24.848  gms.  or  looo  mils  —  V.S. 

N 
Thus  we  see  that  1000  mils  of  —  Na2S203.5H20  represent 


ANALYSIS  BY  OXIDATION  AND   REDUCTION       219 

12.692  gms.  of  iodin,  which  are  equivalent  to  3.546  gms.  of 

chlorin. 

Each  mil  therefore  ^s  equivalent  to  0.003518  gm.  of  chlorin. 

This   number  is  the  factor  v^hich,   v^hen  multiplied  by   the 

N 
number  of  mils  of  —  thiosulohate  used,  gives  the  weight  in 

grams  of  chlorin  contained  in  the  quantity  of  chlorin  water 
acted  upon. 

Chlorinated  Lime.  (CaU.  Chlorinata,  Chlorid  of  Lime, 
Bleaching-powder).  This  substance  was  formerly  supposed 
to  be  a  compound  of  lime  and  chlorin,  CaOCU,  and  hence 
the  name  chlorid  of  Hme.  It  is  now  generally  considered  to 
be  a  mixture  principally  of  calcium  chlorid  and  calcium  hypo- 
chlorite, CaCl2  +  Ca(C10)2  or  Ca(OCl)Cl.  The  hypochlorite 
is  the  active  constituent.  This  is  a  very  unstable  salt,  and 
is  readily  decomposed  even  by  carbonic  acid.  When  treated 
with  hydrochloric  acid  it  gives  off  chlorin. 

The  value  of  chlorinated  lime  as  a  bleaching  or  disinfecting 
agent  depends  upon  its  available  chlorin,  that  is,  the  chlorin 
which  the  hypochlorite  yields  when  treated  with  an  acid. 

In  estimating  the  available  chlorin,  the  latter  is  liberated 
with  acetic  acid.  This  liberated  gas,  then,  acting  upon  potas- 
sium iodid,  sets  free  an  equivalent  amount  of  iodin.  The 
quantity  of  iodin  is  then  determined,  and  thus  the  amount 
of  available  chlorin  found.  0.2  to  0.4  gm.  are  convenient 
quantities  to  operate  upon. 

Introduce  into  a  stoppered  weighing  bottle  between  3 
and  4  gms.  of  chlorinated  lime  and  weigh  accurately.  (In 
order  to  make  the  descriptions  simpler  we  will  assume  that 
3.5  gms.  is  the  weight  taken.)  This  is  triturated  thoroughly 
with  50  mils  of  water,  and  the  mixture  transferred  to  a  graduated 
vessel,  together  with  the  rinsings,  and  made  up  to  1000  mils 
with  water.    This  is  thoroughly  shaken,  100  mils  of  it  (repre- 


220     THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

senting  0.35  gm.  of  the  sample)  are  removed  by  means  of 
a  pipette  and  treated  with  i  gm.  of  potassium  iodid  *  and 
5   mils  of  acetic  acid,  and  into  the  resulting  reddish-brown 

N 
liquid  the  —  sodium  thiosulphate  is  delivered  from  a  burette. 

Towards  the  end  of  the  titration,  when  the  brownish  color 
of  the  liquid  is  very  faint,  a  few  drops  of  starch  solution  are 
added  and  the  titration  continued  until  the  bluish  or  greenish 
color  produced  by  the  starch  has  entirely  disappeared.  Not 
less  than  30  mils  of  the  volumetrtP  solution  sould  be  required 
to  produce  this  result. 

The  reactions  which  take  place  in  this  process  are  illus- 
trated by  the  following  equations : 

Ca(OCl)  CI  +  2HCI  =  CaCl2  +  H2O  +  CI2      * 
or       Ca(OCl)Cl  +  2HC2H302  =  Ca(C2H302)2+H20  +  Cl2, 

Cl2  +  2KI  =  2KCl+l2. 

2)70.92  2)253.84 

10)35-46  10)126.92 

3.546  gms.  12.692  gms. 

I2  +  2(Na2S203  +  5H2O)  =  2NaI  +  Na2S406  +  10H2O. 

2)253.84  • 

10)126.92    -o^  ^      ^ 

I2.6g2  gms.  =1000  mils  —  thiosulphate  V.S. 

It  is  thus  seen  that  i  mil  of  the.  decinormal  sodium  thio- 
sulphate represents  0.012692  gm.  of  iodin,  which  in  turn  is 
equivalent  to  0.003546  gm.  of  chlorin. 

Then 

0.003546X30  =  0.1063  gm. 

o.  1063X100 


0-35 


=  30.37  per  cent  of  available  chlorin. 


*  In  order  to  assume  a  sufficient  excess  of  potassium  iodid,  take  twice 
as  much  of  it  as  of  the  bleaching-powder. 


x\NALYSIS  BY  OXIDATION  AND  REDUCTION       221 

This  is  a  very  rapid  method  for  estimating  chlorin;  but 
when  calcium  chlorate  is  present  in  the  bleaching-powder 
(and  it  often  is,  through  imperfect  manufacture)  the  chlorin 
from  it  is  recorded,  as  well  as  that  from  the  hypochlorite, 
the  chlorate  being  decomposed  into  chlorin,  etc.,  by  hydro- 
chloric acid  (which  is  sometimes  used).  The  chlorate,  however, 
is  of  no  value  in  bleaching;  its  chlorin  is  not  available.  Hence, 
unless  the  powder  is  known  to  be  free  from  chlorate,  the  anal- 
ysis should  be  made  by  means  of  arsenous-acid  solution,  or 
by  using  acetic  acid  instead  of  hydrochloric,  and  thus  avoid 
liberating  chlorin  from  the  chlorate  which  may  be  present 

The  various  bleaching  preparations  of  the  market  which 
depend  upon  their  available  chlorin  are  all  salts  of  hypochlorous 
acid  (HCIO)  or  solutions  of  such  salts. 

Eau  de  Javelle  (Javelle's  Water)  is  a  solution  of  potas- 
sium hypochlorite  and  potassium  chlorid.  A  solution  of  mag- 
nesium hypochlorite  is  known  in  commerce  as  Ramsay s  or 
Grouvelle's  Bleaching  Fluid.  The  solution  known  as  Wilson^ s 
Bleaching  Fluid  contains  aluminum  hypochlorite. 

Solution  of  Chlorinated  Soda  (Labarraque's  Solution). 
This  is  an  aqueous  solution  of  several  chlorin  compounds 
of  sodium,  principally  sodium  chlorid  and  hypochlorite,  con- 
taining at  least  2.4  per  cent  by  weight  of  available  chlorin. 

In  this  solution,  as  in  chlorinated  lime,  it  is  the  available 
chlorin  which  is  estimated.  The  chlorin  is  first  liberated  with 
hydrochloric  or  acetic  acid;  this  then  liberates  iodin  from 
potassium  iodid,  and  the  free  iodin  is  then  determined  by 
standard  sodium  thiosulphate. 

Seven  grams  of  chlorinated  soda  solution  are  mixed  with 
50  mils  of  water,  2  gms.  of  potassium  iodid,  and  10  mils  of 
acetic  acid  are  then  added,  together  with  a  few  drops  of  starch 
solution.  Into  this  mixture  the  decinormal  sodium  thiosul- 
phate is  delivered  from  a  burette  until  the  blue  or  greenish 


222      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

N 
tint  of  the  liquid  is  just  discharged.     Each  mil  of  —  thiosul- 

phate  used  up  represents  0.003546  gm.  of  available  chlorin. 

The  potassium  iodid  should  always  be  added  before  the  acetic 

acid,  so  that  the  chlorin  has  potassium  iodid  to  act  upon  as 

it  is  liberated,  and  thus  loss  of  chlorin  is  obviated. 

Bromin  Water^  or   any  substance  containing  free  bromin 

may  be  assayed  in  exactly  the  same  manner  as  that  described 

for  chlorin  water.     Free  chlorin  must,  however,  be  absent. 

N 
Each  mil  of  —  thiosulphate  solution  represents  0.007992  gm. 

of  bromin. 

Assay  of  Hydrogen  Dioxid  (H202  =  34).  The  iodometric 
method,  which  originated  with  Kingzett,*  is  based  upon  the 
fact  that  iodin  is  liberated  from  potassium  iodid  by  hydrogen 
dioxid,  in  the  presence  of  sulphuric  acid,  and  that  this  libera- 
tion of  iodin  is  in  direct  proportion  to  the  available  oxygen 
contained  in  the  dioxid. 

Then  by  determining  the  amount  of  iodin  liberated,  the 
available  oxygen  is  readily  found. 

H2O2  +  H2SO4  +  2KI  =  K2SO4  +  2H2O  +  I2. 

2)34  ^  2)i_6  2)253-84 

17  =  1  available  0=        8  126.92 

This  shows  that  126.92  gms.  of  iodin  are  liberated  by  17 
gms.  of  absolute  dioxid,  which  are  equivalent  to  8  gms.  of 
available  oxygen. 

N 
Thus   1000   mils  of  —   sodium   thiosulphate  V.S.,   which 

absorb  and  consequently  represent  12.692  gms.  of  iodin,  are 
equivalent  to  1.7  gms.  of  H2O2  or  0.8  gm.  of  available  oxygen. 

*  J.  Chem.  Soc,  1880,  Vol.  2>1^  P-  792- 


ANALYSIS  BY  OXIDATION  AND   REDUCTION       223 

N 
Each  mil  of  this  —  V.S.,  then,  represents  0.0017  g^-  of 

H2O2,  and  0.0008  gm.  of  available  oxygen. 

The  coefficients  for  weight  of  H2O2  and  of  oxygen,  it  is  seen, 
are  identical  with  those  used  in  the  permanganate  process. 
Therefore  the  coefficient  for  volume  is  also  the  same  in  this 
method  as  in  the  other  if  i  mil  be  taken  for  assay. 

The  process  is  carried  out  as  follows:    Take  2  or  3  mils 

of  sulphuric  acid,  dilute  it  with  about  30  mils  of  water,  add 

an  excess  of  potassium  iodid  (about  i  gm.),  and  then  i  mil 

of  hydrogen  dioxid.     After  the  mixture  has  been  allowed  to 

stand  five  minutes,  starch  solution  *  is  added,  and  the  titra- 

N 
tion  with  —  sodium  thiosulphate  begun. 

Note  the  number  of  mils  required  to  discharge  the  blue 
color,  and  multiply  this  number:  by  0.0017  g^-  to  find  the 
quantity,  by  weight,  of  H2O2;  by  0.0008  gm.  to  find  the  weight 
of  available  oxygen;  by  0.57  mil  to  find  the  volume  of  avail- 
able oxygen. 

If  18  mils  are  required,  the  solution  is  of  0.57X18  =  10.26 
volume  strength. 

0.0017X18=0.0306  or  3.06  per  cent  H2O2. 
0.0008X18  =  0.0144  or  1.44  per  cent  of  oxygen. 

With  this  method  the  author  has  always  obtained  satisfactory 
results.  The  lack  of  uniformity  in  the  reaction,  which  is 
frequently  reported,  is  doubtless  due  to  the  use  of  insufficient 
acid  or  to  taking  a  too  concentrated  solution  of  the  dioxid. 

The  best  results  are  obtained  if  the  solution  is  not  more 
than  two  volumes  strength. 

*  Starch  solution  may  be  omitted,  as  the  decolorization  of  the  iodin  is 
distinctly  seen  if  the  beaker  is  placed  upon  a  white  surface. 


224      THE  ESSENTIALS  OF  \'OLL  METRIC  ANALYSIS 

The  sulphuric  acid  used  in  this  assay  must  be  free  from 
sulphurous  acid,  arsenous  acid  and  nitric  acid,  and  the  potas- 
sium iodid  must  contain  no  iodate. 

Distillation  Methods.  Manganates,  chromates,  metallic 
peroxids,  and  a  great  variety  of  substances  containing  oxygen, 
including  antimonic  oxid  and  arsenic  pentoxid,  will,  when 
heated  with  concentrated  hydrochloric,  liberate  an  equivalent 
amount  of  chlorin.     This  is  illustrated  by  the  following  equation : 

Mn02  +  4HCI  =  MnCl2  +  2H2O  +  CI2. 

The  chlorin  which  is  evolved,  is  passed  into  a  solution  of 
potassium  iodid  and  liberates  an  equivalent  of  iodin,  which 
latter  substance  is  then  estimated  by  titration  with  sodium 
thiosulphate  solution.  The  quantity  so  found  is  therefore 
a  measure  of  the  original  substance  and  of  its  oxygen  content. 
The  process  may  be  carried  out  by  means  of  the  apparatus 


Fig.  48. 

devised  by  Bunsen,  Fig.  48,  or  by  that  of  Fresenius,  Fig.  49, 
or  Mohr,  Fig.  50. 

An  accurately  weighed  quantity  of  the  substance  to  be 
analyzed  is  introduced  into  the  round-bottomed  flask  a,  Fig. 
48.  The  flask  is  then  filled  to  about  two-thirds  its  capacity 
with  concentrated  hydrochloric  acid,  and  quickly  connected 
by  means  of  a  short  rubber  tube  with  a  long-bulbed  delivery 


ANALYSIS  BY  OXIDATION  AND   REDUCTION       225 

tube,  b,  which  is  introduced  into  and  extends  to  the  bottom  of 
an  inverted  bulbed  retort,  c.  The  larger  bulb  of  the  retort  is 
filled  to  two- thirds  of  its  capacity  with  a  lo  per  cent  solution 
of  potassium  iodid.  Heat  is  applied  to  the  flask,  and  the  chlorin 
distils  over  into  the  potassium  iodid  solution,  which  becomes 
brownish-red  through  liberation  of  iodin.  The  distillation 
is  continued  until  about  one-third  of  the  acid  fluid  has  passed 
over  or  until  a  peculiar  cracking  sound  indicates  the  absorptiqn 
of  hot  hydrochloric  acid  vapor. 

The  flask,  together  with  its  delivery  tube,  is  then  slowly 
removed,   the  heating,  however,   is  continued  until '  the  tube 


Fig.  49. 


is  entirely  withdrawn,  in  order  to  prevent  the  iodid  solution 
being  drawn  over  into  the  flask.  The  retort  is  then  shaken 
so  that  any  traces  of  chlorin  which  may  have  escaped  absorp- 
tioft,  are  taken  up  and  the  contents  of  the  retort  poured  into 


226      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

a  beaker;  the  retort  and  the  delivery  tube  are  then  rinsed  with 
water  and  the  rinsings  added  to  the  fluid  in  the  beaker,  and 
titration  with  standard  sodium  thiosulphate  begun  immediately. 

It  is  important  that  the  quantity  of  potassium  iodid  is  suffi- 
cient to  keep  the  liberated  iodin  in  solution,  and  the  potas- 
sium iodid  free  from  iodate,  and  the  titration  started  with- 
out delay  to  avoid  liberation  of  iodin  through  action  of  the 
air  upon  the  strongly  acid  potassium  iodid  solution.  When 
all  the  chlorin  has  passed  over  and  hydrochloric  acid  gas 
begins  to  distil,  the  liquid  in  the  retort  is  apt  to  be  drawn  back 
into  the  flask  because  of  the  great  affinity  which  hydrochloric 
acid  gas  has  for  water,  and  the  resultant  condensation  in  the 
flask.  This  regurgitation  may  be  avoided  by  introducing 
into  the  generating  flask  a  small  piece  of  magnesite,  which 
slowly  dissolves  in  the  acid  solution  and  so  keeps  up  a  constant 
flow  of  carbon  dioxid,  which  by  its  pressure  prevents  back-flow 
of  the  fluid.  The  bulbs  in  the  retort  and  delivery  tube  are 
also  calculated  to  prevent  this  regurgitation. 

The  Fresenius  apparatus  is  illustrated  in  Fig.  49.  In  this 
the  potassium  iodid  solution  is  contained  in  two  joined  U-shaped 
tubes.  The  delivery  tube  from  the  distilling  flask  enters  one 
of  the  U-tubes  through  a  paraffin-soaked  cork  (which  fits 
■tightly),  and  terminates  just  above  the  potassium  iodid  solu- 
tion. In  operation  the  U-tubes  should  be  kept  in  ice  water, 
and  all  the  fittings  should  be  air-tight.  Paraffin-covered  cork 
stoppers  only  should  be  used. 

After  all  the  chlorin  has  passed  over  or  when  about  one-third 
of  the  acid  has  distilled  over,  the  apparatus  is  allowed  to  stand 
for  a  few  minutes,  to  permit  all  traces  of  chlorin  to  become 
absorbed;  the  application  of  a  suction  pump  to  the  rear  outlet 
tube  will  help  to  bring  about  this  result. 

Mohr's  apparatus,  showm  in  Fig.  50,  is  of  very  simple  con- 
struction and  easy  to  use. 


ANALYSIS  BY  OXIDATION  AND  REDUCTION       227 

The  distilling  flask  is  fitted  with  a  paraffin-soaked  cork, 
through  which  a  delivery  tube  containing  one  bulb  passes; 
this  delivery  tube  again  passes  through  a  common  cork  which 
loosely  fits  a  stout,  large  test  tube,  containing  the  potassium 
iodid  solution.  The  delivery  tube  is  drawn  out  to  a  fine  point 
and  reaches  to  near  the  bottom  of  the  test  tube.  The  latter 
is  placed,  when  in  operation,  in  a  hydrometer  jar  containing 
cold  water. 

Estimation  of  Manganese  Dioxid  (Mn02  =  86.93).  o-4 
gm.  of  pulverized  manganese  dioxid  is  placed  into  the  distil- 


FiG.  50. 


ling  flask  (a,  Fig.  48  or  Fig.  49)  and  the  latter  filled  to  two- 
thirds  of  its  capacity  with  concentrated  hydrochloric  acid  and 


228      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

connected  without  delay  with  the  vessel  containing  the  potas- 
sium iodid  solution.  The  flask  is  then  gradually  heated  so 
that  a  steady  current  of  chlorin  passes  over  into  the  potassium 
iodid  solution.  When  the  evolution  of  chlorin  gas  begins  to 
diminish,  the  heat  is  slowly  raised  to  boiling,  and  continued 
at  this  point  until  about  one-third  of  the  acid  liquid  has  dis- 
tilled over.  The  delivery  tube  is  then  removed  and  rinsed,  as 
previously  described,  and  the  liberated  iodin  titrated  by  means 

N 
of  —  sodium  thiosulphate  solution,  of  which  we  will  assume 

60  mils  were  consumed. 

The  reactions  are  as  follows: 

{a)  Mn02  +4HCI  =  MnCl2  +  2H2O  +  CI2. 

86.93  70.92 

(b)  Cl2  +  2KI  =  2KCl  +  l2. 

70.92  253.84 

(c)  I2  +  2Na2S203-5H20  =  2NaI  +  Na2S406  +  10H2O. 

2)253-84  2)496.96 

10)126.92         10)248.48 

12.692  gms.       24.848  gms.  =  iooo  mils  —  V.S. 

10 

N 
0.012692  em.  =1  mil  —  V.S. 

10 

These  equations  show  that  494.96  gms.  of  sodium  thiosul- 
phate will  decolorize  253.84  gms.  of  iodin,  which  quantity  is 
liberated  by  70.92  gms.  of  chlorin,  which  is  itself  liberated 
from  hydrochloric  acid  by  86.93  gms.  of  manganese  dioxid. 

Therefore  i  mil  of  a  decinormal  solution  of  sodium  thio- 
sulphate (containing  24.848  gms.  in  1000  mils)  is  equivalent 
to  0.012692  gm.  of  I;  0.003546  gm.  of  CI;  0.0043465  gm.  of 
Mn02;  0.0008  gm.  of  O  (available). 

The  60  mils  of  the  thiosulphate  solution  used  in  this  assay 
will  therefore  represent  0.0043465X60  =  0.2608  gm.  of  pure 
Mn02,  or  65.2  per  cent. 


ANALYSIS   BY  OXIDATION  AND   REDUCTION       229 

0.2608X100 

=  6s-2  per  cent. 

0.4  ^     ^ 

This  is  the  method  which  should  be  used  for  the  assay  of 
native  manganese  dioxid.  The  freshly  precipitated  manganese 
dioxid  of  the  Pharmacopoeia  may  be  assayed  by  the  more 
easily  performed  digestion  method  described  on  page  233. 

Estimation  of  Chromic  Acid  and  Chromates.  Chromic 
anhydrid,  chromium  trioxid  (CrOs^  100),  when  heated  with 
concentrated  hydrochloric  acid,  liberates  chlorin  as  per  the 
equation, 

CrOa  +  6HC1  =  CrCls  +  3H2O  +  CI3. 

100  3X35-46 

One  hundred  parts  of  CrOs  liberates  3X35.46  parts  of  CI, 
hence  one  atomic  weight  of  chlorin,  35.46  parts,  represents  33.3 

N 
+ parts  of  CrOa.     Or  i  mil  of  —  sodium  thiosulphate  repre- 
sents 0.00333  gm.  of  CrOs. 

Estimation  of  Potassium  Bichromate  (K2Cr207  =  294.20). 
This  salt,  as  explained  in  a  previous  chapter,  has  three  atoms 
of  oxygen  available  for  oxidation.  A  molecule  of  this  salt  is 
therefore  equivalent  to  six  atoms  of  chlorin,  and  when  boiled 
with  hydrochloric  acid  will  liberate  six  atoms  of  chlorin,  as 
the  equation  shows. 

K2Cr207  +  i4HCl  =  2KCl  +  2CrCl3  +  7H20+3Cl2. 

294.2  •  6X35.46 

Thus  one  atom  of  liberated  chlorin  will  represent  one-sixth 

of  294.2,  which  is  49.03  +  parts  of  potassium  dichromate.     Then 

N 
I  mil  of  —  sodium  thiosulphate  will  represent  0.004903  gm. 

of  K2Cr207.  In  the  same  way  all  other  chromates  may  be 
treated,  but  these  compounds  will,  when  treated  with  hydro- 
chloric acid,  liberate  chlorin  at  once  and  without  the  applica- 


230      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

tion  of  much  heat,  hence  some  chlorin  is  apt  to  be  lost  before 
the  distillation  flask  can  be  connected  with  the  apparatus,  and 
therefore  it  is  more  convenient  to  employ  the  digestion  method 
later  described. 

The  reaction  in  the  case  of  neutral  potassium  chromate  is 
as  follows: 

K2Cr04  +  8HC1  =-  2KCI +4H2O  +  CrCls  +  CI3. 

194.2 

Lead  peroxid,  Pb02;  cobaltic  oxid,  C02O3;  nickel  oxid, 
Ni203,  as  well  as  many  other  substances,  may  be  assayed  by 
this  distillation  method. 

Estimation  of  Alkali  lodids  by  the  Distillation  Method. 
This  method  is  based  upon  the  fact  that  metallic  iodids,  when 


Fig.  51. 

treated  with  ferric  salts  in  acidulated  solution,  yield  up  all  of 
their  iodin.     As  shown  in  the  equation 

Fe2(S04)3  +  2KI::=K2S04  +  2FeS04  +  l2. 

The  iodin  thus  set  free  is  distilled  into  a  solution  of  potassium 
iodid  and  its  quantity  determined  by  titration  with  sodium 
thiosulphate  in  the  usual  manner. 


ANALYSIS  BY  OXIDATION  AND   REDUCTION       231 

For  the  reaction,  ferric  sulphate  or  ammonio-ferric  alum 
may  be  used;  the  latter  is,  however,  preferred  because  of  its 
paler  color.  Ferric  sulphate  and  ferric  chlorid  are  so  dark  in 
color  that  the  determination  of  the  end-reaction  is  quite  a 
difficult  matter;  furthermore,  these  salts  frequently  contain 
traces  of  nitrates  which,  if  present,  liberate  chlorin  from  the 
chlorids  or  distil  over  and  liberate  iodin  from  the  potassium 
iodid  solution  in  the  receiving  vessel.  Ferric  chlorid  is  par- 
ticularly objectionable  because  of  the  tenacity  with  which  it 
holds  the  last  portions  of  iodin. 

The  distillation  may  be  done  in  the  Fresenius  apparatus, 
Fig.  49,  or  better  in  that  shown  in  Fig.  51.  The  latter  consists 
of  a  loo-mils  distilling  flask  (a),  connected  by  means  of  a  glass 
tube  with  a  nitrogen  flask  (6),  which  contains  a  10  per  cent 
potassium  iodid  solution,  and  which  is  kept  in  a  vessel  of  ice 
water  when  in  use.  The  stoppers  used  are  cork,  well  soaked 
in  paraffin.  The  construction  of  the  flask  is  particularly  suit- 
able, because  it  presents  a  large  surface  to  the  vapor  of  iodin 
which  distils  over,  and  because  the  titration  can  be  done  directly 
in  it,  thus  avoiding  the  necessity  of  transferring  its  contents 
to  a  beaker  or  other  vessel. 

The  glass  tube  which  conveys  the  iodin  vapor  must  not  be 
carried  into  the  solution  of  potassium  iodid  and  must  not  be 
drawn  to  a  fine  point.  The  reason  for  this  is  that  the  iodin 
condensing  at  the  point  would  soon  choke  up  the  tube  and 
prevent  the  further  passage  of  iodin  vapor.  Any  iodin  which 
condenses  in  the  tube  is  washed  down  into  the  potassium  iodid 
solution  by  the  steam  during  the  distillation. 

The  Process.  Into  the  flask  (a)  is  introduced  about  5  gms. 
of  ammonio-ferric  alum,  50  mils  of  water,  20  mils  of  diluted 
sulphuric  acid  (i :  10),  and  the  iodid  to  be  examined,  accurately 
weighed.  Take  about  0.5  gm.  The  flask  is  then  connected 
with  the  receiving  vessel  (&),  which  is  about  half  filled  with  a 


232      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

lo  per  cent  potassium  iodid  solution,  and  after  connections  are 
made  tight,  heat  is  gradually  applied  to  the  distilling  flask. 
After  most  of  the  iodin  has  passed  over,  the  heat  is  raised  to 
boiling,  and  continued  at  this  temperature  until  about  one- 
fourth  of  the  liquid  has  passed  over,  and  the  solution  in  the 
distilling  flask  is  no  longer  of  a  brown  color. 

When  the  receiving  vessel  has  sufficiently  cooled,  it  is  dis- 
connected and  its  contents  titrated  with  decinormal  sodium 
thiosulphate,  using  starch  as  indicator.  Before  beginning 
titration,  however,  it  is  necessary  to  rinse  the  lower  extremity 
of  the  tube  and  the  stopper  into  the  solution  in  the  receiving 
vessel,  in  order  that  every  trace  of  iodin  be  collected. 

The  calculation  is  then  made  as  follows* 

(a)  Fe2(S04)3.(NH4)2S04  +  2KI 

332.04 
=  K2SO4  +  (NH4)  2SO4  +  2FeS04  + 12. 

253-84 

(b)  I2  +  2Na2S203  +  5H2O  =  2NaI  +  Na2S406  +  10H2O. 
2)253.84 

10)126.92  ^    j^ 

12.692  gms.      =1000  mils  —  V.S. 

N 
0.012692  gm.=       I  mil  —  V.S 

Referring  to  the  above  equations  we  see  that  253.84  gms. 
of  iodin  are  liberated  from  332.04  gms.  of  potassium  iodid; 
thus  126.92  gms.  of  iodin  represent  166.02  gms.  of  potassium 
iodid,  and  therefore  i  mil  of  decinormal  sodium  thiosulphate 
solution,  representing  0.012692  gm.  of  iodin,  will  at  the  same 
time  represent  0.016602  gm.  of  KI. 

N 
If  in  the  above  assay  29  mils  of  —  sodium  thiosulphate 

are  consumed,  we  multiply  the  factor  for  KI  =  0.016602  gm. 
by  29,  this  gives  0.4814+  gm.,  the  quantity  of  pure  KI  in  the 
0.5  gm.  taken,  which  is  about  96  per  cent. 


ANALYSIS  BY  OXIDATION  AND  REDUCTION       233 


Digestion  Methods.  The  distillation  methods  above  de- 
scribed may  be  avoided  in  many  cases  and  the  more  easily 
performed  digestion  process  used.  For  instance,  freshly  pre- 
cipitated manganese  dioxid,  lead  peroxid,  chromic  acid,  chlor- 
ates, bromates,  iodates,  ferric  salts,  and  a  great  many  other 
substances,  may  be  assayed  by  mere  digestion  with  hydro- 
chloric acid  at  a  slightly  elevated  temperature. 

The  digestion  is  performed  in  a  strong  glass  bottle,  provided 
with  an  accurately  fitting  ground-glass  stopper  which  is  tied 
down  by  means  of  wire  or  secured  by  a 
clamp.     See  Fig.  52. 

Before  using  the  bottle  for  this  opera- 
tion it  should  be  tested  by  securely  tying 
down  the  stopper  and  immersing  the 
bottle  entirely  in  hot  water  to  see  if  the 
stopper  fits  sufficiently  tight.  If  it  does 
not,  bubbles  of  air  will  escape  from 
inside,  and  the  bottle  is  useless  for  the 
purpose  intended.  In  that  event  the 
stopper  must  be  reground  into  the  neck 
of  the  bottle  with  a  little  very  fine  emery 
and  water.  The  capacity  of  the  bottle  may  vary  from  50  to 
150  mils. 

The  Process.  The  substance  is  accurately  weighed  and 
introduced  into  the  bottle  together  with  a  small  quantity  of 
coarsely  powdered  glass  or  small  pure  flint  pebbles  (to  prevent 
caking,  especially  in  the  case  of  insoluble  powders).  A  suffi- 
cient excess  of  potassium  iodid  solution  is  then  added,  followed 
by  some  pure  concentrated  hydrochloric  acid.  The  stopper 
is  then  quickly  inserted,  firmly  secured  by  wire  or  a  clamp, 
and  the  bottle  placed  in  a  water  bath,  and  the  water  gradually 
heated  to  boiling;  this  temperature  being  continued  until 
decomposition  is  complete,  which  is  usually  in  about  half  an 


Fig.  52. 


234      THE  ESSENTIALS  OF  VOLUMETRIC*  ANALYSIS 

hour.  The  bottle  is  then  allowed  to  cool  slowly  and  its  con- 
tents emptied  into  a  beaker.  Then,  after  washing  the  bottle 
and  adding  the  washings  to  the  contents  of  the  beaker,  the 
liberated  iodin  is  estimated  by  titration  with  sodium  thio- 
sulphate. 

The  potassium  iodid  used  in  this  process  must  be  absolutely 
free  from  iodate. 

N        .  .  .        - 

One  mil  of  —  sodium  thiosulphate  is  equivalent  to 

KCIO3 0.002042    gm. 

NaClOs.-.. 0.001784 

KBrOs 0.0027836 

KIO3 : 0.003567 

Mn02 0.0043465 

Pb02 0.011955 

Cr03 0.00333 

Estimation    of    Chlorates,    Bromates    and    lodates.     The 

estimation  of  these  salts  is  based  upon  the  fact  that  in  each 
case  one  equivalent  of  the  acid  or  its  monobasic  salt  liberates 
six  equivalents   of  chlorin  and  consequently  six  equivalents 
of  iodin  when  decomposed  by  the  digestion  method. 
This  is  illustrated  by  the  equations: 

(a)  KC103  +  6HCl  =  3H20  +  KCl  +  Cl6; 

(b)  KBr03  +  6HCl  =  3H20  +  KBr  +  Cl6; 

(c)  KIO3  +  6HC1  =  3H2O  +  KI  +  Cle. 
and 

(d)  Cle  +  6KI  =  6KCl+l6. 

In  the  distillation  process,  however,  bromates  and  iodates 
liberate  only  four  equivalents  of  iodin,  while  bromous  chlorid 


A      ANALYSIS  BY  OXIDATION  AND   REDUCTION       235 

and  iodous  chlorid  remain  in  the  retort,  therefore  in  these  cases 
the  digestion  is  preferable  to  the  distillation  method. 

If  the  bromate  or  iodate  to  be  assayed  contains  any  bromid 
or  iodid,  bromin  or  iodin  respectively  will  be  liberated  upon  the 
addition  of  the  acid,  according  to  the  equations 

(a)     5KBr  +  KBrOs  +  6HC1  =  6KC1 +3H2O +Br6 ; 

(6)  5  KI  +  KIO3  +  6HC1  =  6KC1  +  3H2O + le ; 

therefore  the  method  is  not  applicable  for  the  assay  of  such 
mixtures. 

The  presence  of  either  of  these  salts  may  be  ascertained 
by  moistening  a  small  quantity  of  the  salt  wdth  dilute  sulphuric 
acid,  when  if  a  yellow  or  bro^Mi  coloration  results,  either  a 
bromid  or  an  iodid  respectively  is  present. 

Example.  Estimation  of  Potassium  Chlorate.  0.2  gm.  of 
the  salt  is  introduced  into  the  digestion  bottle  of  about  100  mils 
capacity,  .10  mils  of  water  added,  and  about  4  gms.  of  potas- 
sium iodid  (or  sufficient  of  its  saturated  solution).  This  is 
followed  by  10  mils  of  concentrated  hydrochloric  acid,  the 
stopper  quickly  inserted,  firmly  secured  by  wiring  or  a  clamp, 
and  the  flask  placed,  stopper  downward,  in  a  water  bath. 
The  water  is  gradually  raised  to  boiling  and  kept  at  this 
temperature  for  about  half  an  hour.  It  is  then  allowed  to 
cool  slowly,  and  the  contents  of  the  bottle  washed  into  a 
beaker   and    titrated    with    decinormal    sodium    thiosulphate, 

N 
using  starch  as  indicator.     The  number  of  mils  of  —  thio- 

10 

sulphate  solution  used,  multiplied  by  0.C02042  gm  gives  the 
weight  of  pure  KCIO3  present  in  the  sample. 

In  the  assay  of  bromates  and  iodates  a  smaller  quantity 
of  hydrochloric  acid  may  be  used,  and  a  lower  temperature, 
say  50°  C^  is  sufficient  for  decomposition. 


236      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

Example.  Estimation  of  Potassium  Br  ornate,  0.2  gm.  of 
the  salt  is  dissolved  in  15  mils  of  water,  4  gms.  of  potassium 
iodid  are  added,  followed  by  4  mils  of  concentrated  hydro- 
chloric acid.  The  bottle  is  securely  closed,  as  in  the  fore- 
going assay,  and  heated  for  half  an  hour  at  50°  C.     Then, 

.  .        ■  N 

after   decomposition,    titration    with     —   sodium    thiosulphate 

solution  is  begun,  each  mil  of  which  represents  0.2836  gm. 
of  pure  KBrOs. 

If  Tc?oro  of  one-sixth  of  the  molecular  weight  of  either  salt 

N* 
be  taken  for  assay,  each  mil  of  the  —  thiosulphate  solution 

used  will  indicate  i  per  cent  purity. 

Estimation  of  Ferric  Salts.  \\Tien  a  ferric  salt  in  an 
acidulated  solution  is  digested  with  an  excess  of  potassium 
iodid jthe  salt  is  reduced  to  the  ferrous  state,  and  iodin  is  set  free. 

FesCle  +  2KI  =  2FeCl2  +  2KCI  + 12. 

One  atom  of  iodin  is  liberated  for  each  atom  of  iron  in 

the  ferric  state.     The  liberated  iodin  is  then  determined  by 

sodium  thiosulphate  in  the  usual  way.     126.92  gms.  of  iodin 

=  55.82  gms.  of  metallic  iron. 

The  method  is  exemplified  in  the  following  assay: 

Ferric    Chlorid    (Fe2Cl6   or   FeCls).     0.5   of   the   dry  salt 

accurately  weighed  is  put  into  a  loo-mil  glass-stoppered  bottle, 

15  mils  of  water  are  added  to  dissolve  the  salt,  and  then  3 

mils  of  concentrated  hydrochloric  acid  and  2  gms.  of  pure 

potassium  iodid  are  introduced.     The  bottle  is  securely  closed 

and  its  contents  heated  to  40°  C.  (104°  F.)  for  half  an  hour. 

A  higher  temperature  must  be  avoided,  otherwise  iodin  will 

be  volatilized.     The  liquid  is  then  cooled  and  the  liberated 

N 
iodin  titrated  with  —  sodium  thiosulphate  until  the  color  is 

just  discharged.     Starch  may  used  as  indicator. 


ANALYSIS  BY  OXIDATION  AND   REDUCTION       237 

N 
Each  mil  of  —  thiosulphate  solution  used  represents  0.005582 

gm.  of  metallic  iron  or  0.01622  gm.  of  pure  ferric  chlorid. 
The  following  equations  illustrate  the  reactions : 

FegCle  +  2KI  =   2FeCl2  +  2KCI   4-  I2. 

2)324-4  2)253.84 

162.2  126.92 

Then 

I2  +  2Na2S203-5H20  =2NaI  +  Na2S406  +  10H2O. 

2)253-84 
10)126.92  j^ 

12.692  gms.  =  1000  mils  —  V.S. 
^    ^  10 

The  assay  of  other  ferric  salts  is  practically  the  same  as 

that  described  above. 

N 
Each  mil.  of —  sodium  thiosulphate  is  equis^alent  to 

Ferrum,  Fe 0.005582  gm. 

Ferric  ammonium  sulphate,Fe2  (SO4)  3.  (NH4)  2SO4  o .  0266  " 

Ferric  chlorid,  Fe2Cl6 0.01622  " 

"      (cryst.),  FeoCl6  +  i2H.20 0.02702  " 

"      nitrate,  Fe2(N03)6 • 0.024x85  " 

"      oxid,  Fe203 0.007982  " 

"      sulphate,  Fe2(S04)3  •  • 0.019992  '' 

In  the  case  of  the  scale  salts  of  iron,  which  are  mostly  of 
indefinite  and  variable  composition,  it  is  the  quantity  of  metallic 
iron  present  which  is  determined. 

The  solutions  of  ferric  salts  are  estimated  in  the  same  man- 
ner.    It  is  the  rule  to  take  i  to  2  gms.  of  the  solution  for  assay. 

Assay  of  Chromic  Acid,  Chromates  and  Dichromates. 
Dissolve  about  i  gm.  of  the  substances  accurately  weighed  in 
a  stoppered  weighing  bottle,  in  sufficient  distilled  water  to 
measure  100  mils. 

To  15  mils  of  this  solution  add  3  mils  of  hydrochloric  acid 


238      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

and  2  gms.  of  potassium  iodid  in  a  glass-stoppered  container, 
shake  the  mixture,  allow  it  to  stand  for  five  minutes,  tb'^n 
dilute  with  distilled  water  to  measure  loo  mils,  and  titrate  with 
tenth-normal  sodium  thiosulphate  V.S.,  using  starch  T.S.  as 
indicator. 

Each  mil  of  the  thiosulphate  V.S.   used   corresponds  to 

^'^^333  g^-  of  CrOa. 

See  equations  on  page  229. 

Assay  of  Arsenic  Oxid  (AS2O5)  and 'Arsenates.  Also  anti- 
monic  compounds.  Dissolve  about  0.5  gm.  of  the  substance 
accurately  weighed  in  25  mils  of  distilled  water,  heat  the  solu- 
tion to  80°  C.  and  add  10  mils  of  hydrochloric  and  3  gms.  of 
potassium  iodid.  Allow  the  mixture  to  stand  for  fifteen  minutes 
at  80°  C,  then  cool  and  titrate  with  tenth-normal  sodium 
thiosulphate  V.S.,  using  starch  as  indicator.  The  solution  must 
be  decidedly  acid  in  reaction.  The  function  of  the  acid  is 
expressed  by  the  equation 

KI-hHCl  =  KCl-FHL 

The  HI  then  reacts  with  the  AS2O5,  reducing  it  to  AS2O3 
and  liberating  iodin,  as  the  following  equation  shows : 

AS2O5 + 4HI  =  AS2O3  -h  2H2O  4-  2I2. 

Each  mil  of  the  tenth-normal  thiosulphate  V.S.  corre- 
sponds to  0.005748  gm.  o|^As265  or  0.0092985  gm.  of  Na2HAs04 
(sodium  arsenate.) 

Assay  of  Copper.  This  method  depends  upon  the  reaction 
between  copper  salt  and  potassium  iodid,  in  which  iodin  is 
quantitatively  liberated. 

2CUSO4  +4  KI  =  CU2I2  +  2K2SO4  +  I2. 
2X159.64  2X126.92 

The  solution  of  the  copper  sulphate  should  be  weakly  acid. 
A.cetic  acid  is  usually  used.     All  iodin  liberating  substances 


ANALYSIS  BY  OXIDATION  AND  REDUCTION       239 

must  be  absent,  as  well  as  all  metals  which  react  with  iodin, 
as  Pb,  As,  Sb,  Bi,  etc. 

The  Process.  About  i  gm.  of  the  copper  salt,  accurately- 
weighed,  is  dissolved  in  50  mils  of  distilled  water;  to  this  is 
added  4  mils  of  acetic  acid  and  3  gms.  of  potassium  iodid. 
The  liberated  iodin  is  then  titrated  with  tenth-normal  sodium 
thiosulphate  V.S.,  using  starch  as  indicator.  Each  mil  of  the 
thiosulphate  V.S.  corresponds  to  0.006357  g^-  metallic  copper 
or  0.015964  gm.  of  CUSO4. 

If  copper  nitrate  is  to  be  assayed,  the  NO  3  ion  must  be 
taken  up  with  ammonia-water,  which  is  added  until  a  clear 
blue  solution  results,  this  is  boiled  for  about  one  minute,  and 
then  acidified  with  acetic  acid,  or  the  solution  is  evaporated 
with  sulphuric  acid  until  the  nitric  acid  is  expelled. 

Assay  of  Mercuric  Salt.  The  mercury  is  precipitated  out 
of  an  alkaline  solution  by  formaldehyde,  and  then  dissolved 
in  an  excess  of  standard  iodin  solution  in  the  presence  of  potas- 
sium iodid.  The  excess  of  iodin  is  then  titrated  with  standard 
'sodium  thiosulphate  V.S. 

Hg  +  l2  =  Hgl2. 

Mercuric  Chlorid  may  be  reduced  to  mercurous  chlorid  by 
means  of  hydrogen  dioxid  in  the  presence  of  tartaric  and  hydro- 
chloric acids.  Frequent  additions  of  hydrogen  dioxid  are 
made  and  the  mixture  warmed  and  filtered.  The  precipitate 
is  washed  and  treated  as  described  under  assay  of  mercurous 
chlorid. 

Assay  of    Mercurous  Chlorid  or  Iodid.    The  mercurous 

N  .     . 
chlorid  or  iodid  is  treated  with  —  iodin  V.S.  and  potassium 

iodid  until  solution  is  complete.    The  reaction  is  as  follows: 
2H^C1  +  6KI 4- 12  =  2HgK2l4  +  2KCI. 


S 


240      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

The  excess  of  iodin  is  then  titrated  with  sodium  thiosul- 
phate,  starch  being  used  as  indicator. 

The  Process.     One  gm.  of  the  salt,  accurately  weighed,  is 

heated  with  lo  mils  of  distilled  water,  50  mils  of  tenth-normal 

iodin  V.S.  and  5  gms.  of  potassium  iodid.     The  flask  is  stoppered, 

and  the'  mixture  allowed  to  stand,  with  occasional  agitation, 

until  complete  solution  has  taken  place.     The  excess  of  iodin 

is  then  titrated  as  above  described. 

N 
Each  mil  of  —  iodin  V.S.  used  corresponds  to  0.023606 

gm.  of  HgCl  or  0.032752  gm.  of  Hgl. 

Reduction  Methods  Involving  the  Use  of  Standard  Arsenous 
Acid  Solution  (Chlorometry) 

As  previously  described,  arsenous  oxid  when  brought  in 
contact  with  iodin  in  an  alkaline  solution,  results  in  an  oxida- 
tion of  the  former  to  arsenic  oxid,  and  a  conversion  of  the 
iodin  to  hydriodic  acid,  as  shown  in  the  equation 

AS2O3  +  2H2O  +  14  =  AS2O  5 + 4HI. 

Advantage  is  taken  of  this  reaction  for  the  estimation,  not 
only  of  arsenous  and  antimonous  compounds,  but  also  of  iodin 
and  the  other  halogens,  chlorin  and  bromin,  as  well  as  of  all 
those  bodies  which,  when  heated  with  hydrochloric  acid,  evolve 
chlorin,  as  for  instance  the  peroxids. 

The  reaction  with  chlorin  is  as  follows: 

CI4  +  2H2O  +  AS2O3  =  4HCI  +  AS0O5. 

This  reaction  is  really  an  oxidation,"  so  far  as  the  formation 
of  arsenic  oxid  (AS2O0)  is  concerned,  but  there  is  no  accom- 
panying reduction.  The  conversion  of  the  halogen  to  an 
haloid  acid  is  not  strictly  a  reduction  in  the  accepted  sense 


ANALYSIS  BY  OXIDATION  AND   REDUCTION       241 

of  the  word.  Nevertheless,  for  obvious  reasons,  we  speak 
of  analyses  done  by  means  of  arsenous  acid  as  reduction 
methods. 

The  chief  value  of  this  method  is  foimd  in  the  estimation 
of  free  chlorin,  as  in  chlorin  water,  and  the  available  chlorin 
existing  in  hypochlorites  or  that  evolved  from  hydrochloric 
acid  by  heatmg  with  peroxids.  Hence  the  designation  "  chlor- 
ometry." 

In  carrying  out  this  method  free  alkali  must  be  present 
to  combine  with  the  haloid  acid  which  is  formed.  The  alkali 
must  be  in  the  form  of  bicarbonate.  Normal  carbonates  or 
hydroxids  are  not  suitable  (see  page  194). 

The  solutions  required  are: 

Decinormal  iodin  (see  page  188); 

Decinormal  arsenous  acid; 

Starch  solution  (see  page  192);  or  iodized  starch  test  paper. 

N 
Preparation  of  Decinormal   —    Arsenous  Acid  (As203  = 

N 
197.92;  —  V.S.  =  4.948  gms.  in  i  liter).      4.948  gms.  of  the 

purest  sublimed  arsenous  anhydrid  (AS2O3)  are  dissolved  in  a 
minimum  of  concentrated  sodium  hydroxid  solution.  When 
complete  solution  is  effected  add  100  mils  of  distilled  water  and 
neutralize  the  excess  of  alkali  with  diluted  sulphuric  acid, 
using  phenolphthalein  as  indicator.  Add  to  this  20  gms.  of 
pure  sodium  bicarbonate  dissolved  in  500  mils  of  distilled  water, 
and  finally  dilute  to  icoo  mils  at  standard  temperature.  It  is 
then  standardized  with  decinormal  iodin,  using  starch  as  indi- 
cator. Decinormal  arsenous  acid  solution  should  correspond, 
volume  for  volume,  with  decinormal  iodin  solution. 

If  this  solution  is  made  from  pure  arsenous  acid,  it  will 
hold  its  titer  for  years,  but  if  any  sulphur  is  present  there  will 
be  an  absorption  of  oxygen  from  the  air  and  a  consequent 


242     THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

oxidation  to  arsenic  oxid.  If  the  presence  of  sulphur  is  sus- 
pected, the  solution  should  be  tested  with  silver  nitrate,  when 
its  presence  will  be  indicated  by  the  formation  of  a  reddish 
precipitate. 

Iodized  Starch  Test  Paper.  A  portion  of  starch  solution 
is  mixed  with  a  few  drops  of  potassium  iodid  solution  and 
in  this  are  soaked  strips  of  pure  white  filtering  paper.  This 
test  paper  is  used  in  the  damp  state;  it  is  then  far  more 
sensitive. 

Estimation  of  Free  Halogens.  The  estimation  of  chlorin, 
bromin  or  iodin  by  the  chlorometric  method  depends,  as 
before  stated,  upon  their  power  of  oxidizing  arsenous  acid. 
When  a  free  halogen  is  brought  in  contact  in  alkaline  solution 
wdth  arsenous  acid,  the  latter  is  oxidized  to  arsenic  acid,  while 
the  halogen  is  transformed  into  a  haloid  acid,  as  per  equations 


CUl 

f  4HCI 

Br4 

+  2H20+As203=As208+     4HBr. 

I J 

4HI 

The  estimation  may  be  carried  out  in  two  ways:  ist,  by 
direct  titration  with  a  standard  arsenous  oxid  solution,  using 
iodized  starch  test  paper  as  indicator;  2d,  by  residual  titra- 
tion, an  excess  of  the  standard  arsenous  oxid  being  taken,  and 
then  retitrating  with  standard  iodin  solution,  using  starch  as 
indicator.  The  residual  titration  method  need  not  be  employed 
for  free  iodin,  as  this  can  be  titrated  direct  with  the  arsenous 
oxid  solution,  using  starch  as  indicator.  Furthermore,  iodin 
need  not  be  brought  into  solution  to  be  titrated  by  this  method. 

The  estimation  of  free  halogens  by  the  direct  chlorometric 
method  is  as  follows: 

An  accurately  weighed  quantity  of  substance  made  alka- 
line by  the  addition  of  sodium  bicarbonate  is  titrated  with 
decinormal  arsenous  acid  solution,  and  from  time  to  time 


ANALYSIS  BY  OXIDATION  AND   REDUCTION       24?, 

during  the  titration  a  drop  of  the  solution  is  removed  on  the 
end  of  a  pointed  glass  rod  and  brought  in  contact  with  a  piece 
of  iodized  starch  test  paper.  So  long  as  free  chlorin  or  bromin 
is  present  the  liquid  will  cause  a  blue  stain  on  the  test  paper, 
but  when  the  halogen  is  all  taken  up  no  blue  color  is  produced. 

If  the  exact  point  is  overstepped  the  resi(Jtial  method  must 
be  used.  A  little  additional  excess,  of  the  ar^enous  acid  solu- 
tion may  be  added,  together  with  a  few  drop^  of  starch  solu- 
tion, and  the  excess  then  titrated  by  means  of  decinormal 
iodin  solution  until  the  blue  color  is  produced.  The  volume 
of  decinormal  iodin  solution  so  used,  deducted  from  the  total 
volume  of  arsenous  acid  solution  taken,  gives  the  exact  quan- 
tity which  was  oxidized  by  the  halogen,  and  from  this  the 
percentage  of  chlorin  or  bromin  may  be  calculated. 

Example  i.  Estimation  of  Chlorin  in  Chlorin  Water, 
Twenty  mils  of  chlorin  water  (sp.gr.  i.o)  titrated  by  the  direct 
method  required  22  mils  of  decinormal  arsenous  acid  solution 
before  the  iodized  starch  test  paper  indicated  the  completion 
of  the  reaction. 

By  referring  to  the  equation  we  see  that  each  mil  of  the 
arsenous  acid  solution  represents  0.003546  gm.  of  chlorin. 
Therefore,,  if  22  mils  were  used,  the  20  mils  of  chlorin  water 
must  have  contained  22X0.003546  gm.  of  chlorin,  which  is 
0.078012. 

The  20  mils  of  chlorin  water  (sp.gr.  i.o)  weigh  20  gms. 

Hence 

0.078012  X 100 

=  39.0+  percent. 

Example  2.  The  20  mils  of  chlorin  water  weighing  20 
gms.  were  treated  with  26  mils  of  the  arsenous  acid  solution, 
starch  solution  was  then  added,  and  the  excess  of  arsenous 
acid  solution  titrated  by  means  of  decinormal  iodin.    Four 


244      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

mils  of  the  latter  were  required,  then  4  from  26  mils  leaves 

N 
22  mils,  the  quantity  of  the  —  arsenous  acid  solution  which 

reacted  with  the  chlorin.  The  calculation  is  the  same  as  in 
Example  i. 

The  reaction  is  as  follows: 

CI4  +  2H2O   +  AS2O3  =  4HCI   +  AS2O5. 

4)141-84  4)197-92 

10)35-46  ^  10)49-48  j^ 

3.546  gms.=  4-948  gms.  =  1000  mils  —  V.S. 

10 

N 
0.003546  gms.  =         I  mil  —  V.S. 

Estimation    of   Available    Chlorin    in   Bleaching   Powder. 

Three  and  one-half  gms.  of  the  bleaching  powder  (chlorin- 
ated lime)  are  triturated  thoroughly  with  50  mils  of  water, 
and  the  mixture  transferred  to  a  graduated  vessel,  together 
with  the  rinsings,  and  made  up  to  loco  mils  with  water. 
This  is  thoroughly  shaken.  100  mils  of  it  (representing  0.35 
gm.  of  the  sample)  is  removed  by  means  of  ,a  pipette  and 
titrated  with  decinormal  arsenous  acid  solution,  as  described 
in  the  foregoing  assay,   using  either  the  iodized  starch  test 

.      .        ,  ,  N 

paper  as  mdicator    or    retitratmg  the  excess  of  —r  arsenous 

N 
acid  solution  added  by  means  of  —  iodin  solution. 

•^  10 

N 
Fach  mil  of  —  AS2O3  V.S.  represents  0.003546  gm.  of 

available  chlorin. 

Ca(OCl)Cl   +  AS2O3  =  AS2O5  +  2CaCl2. 

4)141-84  4)197-92 

•10)35.46  10)49.48  j^ 

3.546  gms.=         4.948  gms.  or  looomils  —  V.S. 

As  seen  by  referring  to  the  above  equation  this  process 
determines  the  value  of  the  chlorinated  lime  by  measuring 


ANALYSIS  BY  OXIDATION  AND  REDUCTION       245 

the  amount  of  arsenous  acid  which  the  oxygen  present  in 

the  active  constituent   (Ca(OCl)Cl)   is  capable  of  oxidizing. 

In  the  formula  of  this   compound  there  are  two  atoms   of 

chlorin  and  one  atom  of  oxygen.     Therefore  the  quantity  of 

bleaching  powder  which  yields  35.46  parts  of  available  chlorin 

will  also  supply  8  parts  of  oxygen;    this  may  therefore  be 

taken  as  the  measure  of  the  chlorin.     The  same  method  may 

be  employed  for  the  assay  of  all  other  solutions  containing 

available  chlorin. 

Assay  of  Manganese  Dioxid  (Chlorometric) .     The  chloro- 

metric  assay  of  manganese  dioxid,  as  well  as  that  of  all  other 

bodies  which  liberate  chlorin  when  heated  with  hydrochloric 

acid,  may  be  naade  in  similar  manner  to  that  described  for 

the  iodometric  assays  of  these  substances,  the  same  apparatus, 

etc.,  being  used. 

...  N 

The  liberated  chlorin  is,  however,  titrated  with  —  arsenous 

10 

acid  solution. 

The  chlorin  may  be  distilled  into  a  solution  of  sodium 

N 
carbonate,  and  this  solution  is  then  titrated  with  —  arsenous 

10 

acid  or  the  chlorin  may  be  distilled  directly  into  a  measured 

N 
volume  of  —  arsenous  acid  solution  and  the  latter  then  titrated 
10 

N  .     . 
with  —  iodin  solution,  using  starch  as  indicator,  the  difference 
10  '         &  5 

between  the  volume  of  iodin  solution  used  and  that  of  the 
arsenous  acid  solution  taken  is  the  measure  of  the  latter  which 
reacted  with  the  chlorin. 

It  is  a  good  plan  in  each  case  to  divide  the  solution  into 
two  or  three  equal  parts  and  to  titrate  each  separately. 


246      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

Reduction  Methods  Involving  the  Use  of  Stannous  Chlorid 

Stannous  chlorid  (SnCl2)  is  a  very  powerful  reducing 
agent.  Its  action  in  this  respect  depends  npon  its  affinity 
for  chlorin  which  it  readily  abstracts  from  most  other  chlorids. 
In  its  action  upon  mercuric  chlorid  a  portion  of  the  latter 
is  always  reduced  to  the  metallic  state. 

This  reducing  action  of  stannous  chlorid  is  utilized  in 
certain  volumetric  processes,  especially  in  the  estimation  of 
iron.  In  this  case  it  possesses  an  advantage  over  perman- 
ganate, in  that  the  iron  must  be  in  the  ferric  state,  in  which 
condition  it  is  most  usually  found,  while  if  permanganate 
is  used,  a  preliminary  reduction  to  the  ferrous  state  is  necessary 
before  titrating.  The  great  disadvantage,  however,  is  in  the 
fact  that  even  short  contact  with  air  will  quickly  oxidize  it, 
and  thus  spoil  its  titer.  In  consequence  of  this  it  must  be 
frequently  tested,  and  can  be  used  only  in  the  form  of  empirical 
solutions. 

It  is  particularly  useful  in  the  titration  of  ferric  salts, 
which  salts  can  be  accurately  estimated  by  direct  titration 
with  it,  the  end-point  being  recognized  by  the  disappearance 
of  the  yellow  color  of  the  ferric  solution.  These  salts  may 
also  be  estimated  residually  by  adding  an  excess  of  stannous 
chlorid  solution  of  known  strength  and  retitrating  the  excess 
by  means  of  standard  iodin,  using  starch  as  indicator. 

The  reactions  are: 

''     FesCle  +  SnClo  =  2FeCl2  +  SnCLi 
and 

'^-      SnCl2  +  2HCl+l2  =  SnCl4-f2HL 

The  Estimation  of  Iron  by  Means  of  Stannous  Chlorid 
Solutions  may  be  accurately*  affected  by  the  following  pro- 
cedure, as  suggested  by  Fresenius.  The  solutions  necessary 
are: 


ANALYSIS  BY  OXIDATION  AND   REDUCTION       247 


(a)  A  solution  of  ferric  chlorid  containing  lo  gms.  of  pure 
iron  in  a  liter. 

This  is  made  by  dissolving  10.04  gms.  "of  thin  *  annealed 
binding  wire  (which  contains  99.6 
per  cent  of  pure  iron)  in  a  sufficient 
quantity  of  pure  hydrochloric  acid. 
A  small  quantity  of  potassium 
chlorate  is  then  added  to  effect 
complete  oxidation  of  the  iron,  and 
the  excess  of  chlorin  expelled  by 
boiling.  This  solution  is  then 
cooled  and  diluted  to  i  liter. 

(6)  A  solution  of  stannous  chlo- 
rid made  by  dissolving  about  10 
gms.  of  pure  tin  in  200  mils  of 
strong,  pure  hydrochloric  acid. 
This  may  be  done  by  heating  the 
tin  in  small  pieces  with  the  acid  in 
a  flask,  and  introducing  a  few 
pieces  of  platinum  foil  to  excite 
galvanic  action.  The  solution  so 
obtained  is  diluted  to  about  one  liter  w^ith  distilled  water  and 
should  be  preserved  in  a  bottle,  such  as  shown  in  Fig.  53, 
to  which  air  can  only  gain  access  through  a  strongly  alkaline 
solution  of  pyrogallic  acid.  WTien  so  kept  the  strength  of 
the  solution  can  be  preserved  for  several  weeks. 

(c)  A  solution  of  iodin  in  potassium  iodid.     This  may  be 
approximately  or  exactly  decinormal. 

The  procedure  is  as  follows: 

ist.  The  relation  between  the  tin  solution  and  the  iodin 
solution  is  found. 

2d.  The  relation  between  the  tin  solution  and  the  iron 
solution  is  determined. 


Fig.  53. 


248      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

3d.  The  assay. 

The  relation  between  the  tin  solution  and  the  iodin  solution 
is  found  as  follows  : 

Two  mils  of  the  tin  solution  are  put  into  a  beaker,  a  little 
starch  solution  added,  and  the  iodin  solution  then  delivered  in 
from  a  burette  until  the  blue  color  occurs.  If  4  mils  are  used, 
then  each  2  mils  of  iodin  solution  represents  i  mil  of  tin  solution. 

The  relation  between  the  tin  solution  and  the  iron  solution 
is  found  as  follows : 

Fifty  mils  of  the  iron  solution  (representing  0.5  gm.  of  iron, 
are  put  into  a  small  flask  together  with  a  little  hydrochloric  acid 
and  heated  to  gentle  boiling.  The  tin  solution  is  then  delivered 
from  a  burette  until  the  yellow  color  of  the  iron  solution  is  nearly 
discharged.  It  is  then  added  continuously,  drop  by  drop,  until 
the  color  is  entirely  gone.  Assuming  that  35  mils  were  re- 
quired, then  each  35  mils  of  tin  solution  are  equivalent  to  0.5  gm. 
of  pure  iron.  If  the  end-point  is  not  clearly  recognized  and  an 
excess  of  the  tin  solution  was  added,  the  solution  should  be  quickly 
cooled,  a  few  drops  of  starch  solution  added,  and  the  excess 
estimated  by  titrating  with  the  iodin  solution,  each  mil  of  which 
represents  0.5  mil  of  the  tin  solution.  The  excess  so  found,  de- 
ducted- from  the  total  quantity  of  tin  solution  added,  gives  the 
quantity  of  the  latter,  which  corresponds  to  0.5  gm.  of  iron. 

Having  determined  these  data,  the  analyst  can  readily 
estimate  any  unknown  quantity  of  iron  in  solution  in  the  ferric 
state. 

If  the  iron  is  partly  or  wholly  in  the  ferrous  state  it  may 
be  oxidized  by  adding  some  potassium  chlorate  and  boiling 
to  expel  excess  of  chlorin. 

The  Assay.  A  solution  of  iron  taken  for  analysis,  required 
24  mils  of  the  tin  solution.  The  quantity  of  iron  present  is 
calculated  by  proportion  as  follows : 

35  mils  :  0.5  gm.:  124  mil;  x;        ^^  =  0.34  gm. 


ANALYSIS  BY  OXIDATION  AND  REDUCTION       249 

To  secure  accurate  results  the  iron  solution  assayed  must 
be  fairly  concentrated,  because  then  the  end-reaction  is  more 
readily  seen,  and  also  because  the  greater  the  dilution  the 
larger  the  amount  of  tin  solution  will  be  required.  It  is  good 
policy  to  use  very  little  excess  of  the  tin  solution,  so  that  only 
a  very  small  quantity  of  iodin  solution  is  required. 

Estimation  of  Mercuric  Salts  (Laborde).  This  depends 
upon  the  fact  that  stannous  chlorid  solution  added  to  a  solution 
of  a  mercuric  salt  reduces  the  latter  first  to  mercurous  chlorid 
(calomel)  and  finally  the  calomel  to  metallic  mercury.  The 
reduction  to  calomel  results  in  the  formation  of  a  white  pre- 
cipitate, and  when  the  mercuric  salt  is  completely  reduced 
the  stannous  chlorid  acts  upon  and  reduces  the  calomel  to 
metallic  mercury,  which  results  in  the  production  of  a  char- 
acteristic brownish  color. 

The  reactions  are  as  follows: 

SnCl2  +  2HgCl2  =  SnCU  +  2HgCl ; 
SnCl2  +  2HgCl  =  SnCU  +  Hgs. 

According  to  Laborde  the  tin  solution  is  made  by  dissolving 
8  gms.  of  pure  tinfoil  by  means  of  heat  in  loo  mils  of  pure 
hydrochloric  acid,  and  diluting  to  2  liters. 

This  tin  solution  is  checked  against  a  solution  of  mercuric 
chlorid  containing  10  gms.  per  liter.  To  counteract  the  hin- 
dering effect  of  the  hydrochloric  acid  the  solution  under  anal- 
ysis, containing  o.i  gm.  of  mercuric  chlorid,  is  mixed  with  5 
mils  of  a  solution  containing  100  gms.  of  ammonium  acetate 
and  100  mils  of  acetic  acid  to  the  liter.  The  acetic  acid  pro- 
motes the  disappearance  of  the  brown  color  which  occurs  at 
the  point  where  the  tin  solution  is  in  excess,  but  before  reduc- 
tion is  complete.  The  titration  with  the  tin  solution  is  con- 
tinued until  a  permanent  brown  color  occurs. 


250      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

If  the  brown  color  is  too  dark  from  overstepping  of  the 
end-point,  the  addition  of  i  mil  of  the  mercuric  chlorid  solution 
will  render  the  solution  white  again,  and  the  titration  can 
then  be  carried  further. 

This  method,  which  is  convenient,  rapid,  and  very  accurate, 
can  be  employed  in  many  cases.  If  the  mercuric  solution 
contains  any  free  mineral  acids,  the  latter  must  be  neutralized 
with  ammonia  (in  the  presence  of  ammonium  acetate,  to 
prevent  formation  of  ammoniated  mercury). 

The  presence  of  alkali  and  alkali  earth  salts  or  most  salts 
of  other  metals  (except  iron,  gold,  and  platinum)  do  not 
in  the  least  interfere  with  the  accuracy  of  the  results.  The 
same  is  true  of  organic  acids,  either  free  or  in  combination 
with  alkali. 


PART  II 


CHAPTER  XI 
ESTIMATION  OF  ALKALOIDS 

In  making  alkaloidal  assays  of  drugs  it  has  long  been  the 
custom  to  evaporate  the  final  ethereal  or  chloroformic  extract, 
and  to  v/eigh  the  residue  as  alkaloid.  This  residue  seldom, 
if  ever,  consists  of  the  pure  alkaloid,  and  the  impurities — i.e., 
non-alkaloidal  matter — is  variable  in  amount  and  difficult  to 
entirely  remove,  consequently  gravimetric  results  are  in  many 
cases  very  wide  of  the  truth,  and  hence  unreliable. 

The  volumetric  methods  are  in  most  cases  much  more 
satisfactory. 

While  the  results  of  the  titration  of  the  total  alkaloids  of 
drugs  cannot  be  called  absolutely  accurate,  nevertheless  experi- 
ence has  shown  that  they  are  nearer  the  truth  than  those 
obtained  by  the  gravimetric  method. 

In  estimating  an  alkaloid  by  titration  it  is  essential  to 
know  the  formula  and  molecular  weight  of  the  alkaloid,  as 
well  as  the  equivalent  of  acid  with  which  it  will  combine. 

The  quantity  of  alkaloid  present  is  easily  calculated  when 
we  know  that  a  molecular  weight  of  a  monobasic  acid  or  half 
a  molecular  weight  of  a  dibasic  acid  will  combine  with  and 
neutralize  a  molecular  weight  of  an  alkaloid,  provided  the 
alkaloid  is  a  monacid  base.  If  the  alkaloid  is  a  diacid  base, 
one  molecular  weight  will  combine  with  two  molecules  of  a 
monobasic  acid  or  one  molecular  weight  of  a  dibasic  acid. 

251 


252       THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

Sparteine  and  emetine  (?)  are  diacid  alkaloids;    most  of 
the  others  are  monacid  bases. 
Examples.     Monacid  alkaloids: 

C20H24N2O2  +  HCl  =  C20H24N2O2 .  HCl ; 

Quinine 

(Cl7Hi9N03)2+H2S04=  (Ci7Hi9N03)2.H2S04; 

Morphine 

.     (C2lH22N202)2+H2S04=(C2lH22N202)2-H2S04; 

Strychnine 

Diacid  alkaloids: 

C15H26N2  +  2HCI  =  Ci5H26N2(HCl)2; 

Sparteine 

C30H40N2O5  +  H2SO4  =  C30H40N2O5  -  H2SO4. 

Emetine  (Kunz) 

The  quantity  of  alkaloid  present  in  the  substance  is  easily 
calculated,  as  illustrated  by  this  equation: 

C20H24N2O2  +  HCl  =  C20H24N2O2 .  HCl. 

Quinine 

N 
324.2  gms.    36.46  gms.  =  1000  mils  —  V.S.; 

N 
32.42  **         3.646  "  =1000  mils  — V.S. 

10 

N 
Thus  I  mil  of  —  V.S.  =  0.03242  gm.  of  quinine. 

Ci5H26N2  +  2HCl  =  Ci5H26N2(HCl)2. 

Sparteine 

2)234-2  2)72.92  j^ 

1 1 7.1  gms.        36.46  gms.  =  1000  mils  —  V.S.; 

1 1. 71  "           3.646  "    =  1000  mils  —  V.S. 
10 

N 
One  mil  of  —  V.S.  hence  =  0.01 171  gm.  of  sparteine. 

N 
Thus  1000  mils  of  —  hydrochloric  acid  will  combine  with 


ESTIMATION  OF  ALKALOIDS  .  253 

iV  of  the  molecular  weight  in  grams  of  a  monacid  alkaloid, 
or  -gV  of  the  molecular  weight  of  a  diacid  alkaloid. 

In  the  case  of  drugs  where  two  or  more  alkaloids  are 
present,  accurate  results  can  only  be  obtained  by  determining 
how  much  of  each  alkaloid  is  present  by  a  separate  assay. 
But  often  it  is  assumed  that  the  alkaloids  are  present  in  equal 
quantities,  and  the  mean  of  their  molecular  weights  is  taken 
as  the  basis  for  the  calculation. 

It  must  be  borne  in  mind,  however,  that  in  titrating  alka> 
loids  the  greatest  care  must  be  exercised  and  all  precautions 
closely  observed  in  order  to  attain  any  degree  of  accuracy. 
The  volumetric  solutions  must  be  prepared  with  the  greatest 
care  and  must  be  absolutely  accurate.  The  eye  must  be 
trained  (as  it  can  only  be  through  practice) ,  to  ■  distinguish 
the  end-colors  of  the  indicators  employed,  a  matter  of  some 
difficulty.  Furthermore,  all  measuring  instruments  used  must 
be  accurate,  or  they  should  be  carefully  calibrated  in  order 
to  find  the  necessary  factor  for  correction. 

The  Volumetric   Solutions   usually   employed  in   titrating 

N    N    N    N  N 

alkaloids  are  — ,  — ,  — ,  — ,  and  — .     The  weaker  solutions 
lo   20    25    50  100 

give  more  accurate  results,  as  will  be  understood  if  we  remem- 

N  N 

ber  that  a  —  is  10  X  stronger  than  a  —  and  100  X  stronger 

N  N 

than  —  V.S.     Then  if  —  solution  be  used,  one  drop  may 
100  I  ^  f        J 

overstep  the  neutral  point,  while  if  the  same  solution  were 

N 
treated  with  —  solution  5  drops  would  be  required  to  neutralize, 

N 
which  is  equivalent  to  using  one  half  a  drop  of  —  solution.     A 

N 

solution  will  of  course  be  capable  of  even  more  delicate 

100  ^ 


I 


254       THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

N 
work.    In  the  case  Just  mentioned  45  drops  of  the  —  may 

exactly  neutralize  the  solution,  hence  less  than  half  a  drop 

N  N 

of  —  and  between  4  and   5  drops  of  —  are  represented. 

Therefore  in  all  delicate  alkaloidal  titrations  weak  standard 

solutions  should  be  used — in  fact,  in  all  titrations  where  great 

accuracy  is  required. 

//  the  alkaloid  he  from  a  recent  extraction  and  is  in  the 

form  of  a  free  alkaloid,  it  is  dissolved  in  a  measured  volume 

N 
of  —  acid  solution  and  the  excess  of  acid  solution  then  deter- 
10 

N 
mined  by  residual  titration  with  —  alkali  solution. 
•'  100 

N 
In  this  the  —  sulphuric  acid  solution  is  preferred,  except 

N 
in  the  case  of  quinine  or  cinchonine,  in  which  —  hydrochloric 

acid  gives  better  results. 

The  process  in  detail  is  as  follows:    Place  2  gms.  of  the 

.         N  .       . 

alkaloid  into  a  beaker,  add  75  mils  of  —  sulphuric  acid  solu- 
tion, and  wa];m  on  a  water-bath  until  the  alkaloid  is  com- 
pletely dissolved.  The  solution  is  then  allowed  to  cool  and 
diluted  to  100  mils. 

Ten  mils  of  the  solution  (containing  0.2  gm.  of  the  alkaloid 

N 
and  7.5  mils  of  —   sulphuric  acid  solution)  are  removed  by 

.      N  . 

means  of  a  pipette  and  retitrated  with  —  potassium  hydroxid 

N 
solution.     One-tenth  of  the  quantity  of  the  —  alkali  used 

^  -^  100 

N       .    ■ 
is  deducted  from  the  7.:;  mils  of  the  —  acid  solution,  and  the 
'  ^  10 


ESTIMATION  OF  ALKALOIDS  255 

remainder  is  the  quantity  of  the  latter  which  combined  with 
and  hence  represents  the  alkaloid  present.  This,  if  multiplied 
by  the  factor,  gives  the  weight  of  the  alkaloid. 

Either  haematoxylin  solution  or  Brazil-wood  T.S.  may  be 
employed  as  the  indicator. 

Lf  the  alkaloid  is  soluble  in  alcohol,  as  are  quinine  and 
rodeine,  it  may  be  treated  as  follows: 

Place  2  gms.  of  the  alkaloid  in  a  graduated  cylinder,  dis- 
solve in  alcohol,  and  dilute  the  solution  up  to  loo  mils  with 
alcohol.  Remove  lo  mils  of  this  solution  (containing  0.2  gm. 
of  the  alkaloid)  and  place  in  a  beaker,  add  the  indicator  and 
run  in  the  decinormal  acid  solution  to  slight  excess.  Rotate 
the  beaker  several  times,  let  stand  for  a  few  minutes,  wash 
down  the  sides  of  the  beaker  with  distilled  water,  using  about 

N 
40  mils,  and  retitrate  the  excess  of  acid  with  —  potassium 

100  ^ 

hydroxid  solution,  until  end-color  is  given  by  the  indicator. 

N' 
Deduct  one-tenth  of  the  quantity  of  the  —  solution  used 

N 
from  the  quantity  of  —  acid  solution  added,  and  the  remainder 

is  the  quantity  of  the  latter,  which  combined  with  and  hence 
represents  the  alkaloid  present. 

The  indicators  best  suited  for  most  alkaloids  are  HcBfna- 
toxylin,  Brazil-wood,  Cochineal,  lodeosin  and  Litmus, 

The  color  changes  produced  by  the  principal  indicators 
used  in  alkaloidal  titrations  are  as  follows: 


256       THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 


Haematoxylin  .  . 
Brazil-wood  .  .  . 

Cochineal 

Litmus 

lodeosin 

Lacmoid 

Fluorescein 

Congo  red 

Melhyl-orange  . 
Phenolphlhalein 


Acid. 


Alkali. 


Yellow 

Blue 

" 

Purplish-red 

Yellowish-red 

Purplish 

Red 

Blue 

Yellow 

Rose-red 

Red 

Blue 

Green  fluorescence 

No  fluoresc,  yellowish 

Blue 

Red 

Red 

Straw-yellow 

Colorless 

Red 

TABLE    SHOWING    THE    FACTOR    FOR    VARIOUS    ALKALOIDS 

WHEN  TITRATING  WITH  —  ACID  OR  ALKALI 

lo 


Name. 


Formula. 


Molecular 
Weight. 


Factor. 


Aconitinc 

Atropine 

Brucine 

Cephaeline 

Cinchona  alkaloids  (combined) 

Cinchonine 

Cinchonidine 

Cocaine 

Codeine 

Coniine 


Emetine 


Hydrastine 

Hyoscine 

Hyoscyamine 

Ipecac  alkaloids  (combined) 

Morphine 

Nicotine 

Physostigmine 

Pilocarpine 

Quinine 

Sparteine 

Strychnine 


C34H47NOii 

C17H23NO3 
C23H26N2O4 
Ci^HigNQz 


C19H22N2O 
C19H22N2O 

Ci;H2iNO, 


C3oH44N204(Glenard) 
C30H40N2O5  (Kunz) 
C15H21NO2  (U.  S.  P.) 

CziHojN  Og 

C17H21N04 
C17H23N03 


C17H.9N03 
C5H7N 

C15H22N302 

C11H16N202 

C20H24N2O2 

C15H26N2 


645.48 

289.24 

394.28 
233.20 


294.24 
294.24 
303.22 
299.22 

127.18 
496.32 
508.34 

247.22 

383.22 
303-18 

289.24 


285 .  20 
81.06 
275.24 
208.18 
324.26 
234.28 
334-24 


0.064548 
0.028924 
0.039428 
0.023320 

0.03069 

0.029424 

0.029424 

0.030322 

0.029922 

O.OI27I8 

0.024818 

0.C254I7 

0.024722 

0.038322 
0.030318 

0.028924 

0.024021 

0.02852 

0.008106 

1.027524 

0.020818 

0.032426 

O.OII7I4 

0.033424 


ESTIMATION  OF  ALKALOIDS  257 

gordin's  modified  alkalimetric  method,  using  phenol- 
phthalein  as  indicator 

The  alkaloidal  residue  obtained  by  any  of  the  extraction 

N 
methods  in  use  is  dissolved  in  a  measured  excess  of  —  hydro- 

20     •' 

chloric  acid  solution.  Wagner's  or  Mayer's  reagent  is  then 
added,  a  little  at  a  time,  with  frequent  shaking,  until  the 
alkaloids  are  completely  precipitated.  The  mixture  is  then 
diluted  with  water  to  100  mils  and  shaken  until  the  double 
salt  of  the  alkaloid  and  reagent  completely  separate.  ^Vhen 
allowed  to  stand  a  few  minutes  the  supernatant  liquid 
should  be  clear,  and  if  Wagner's  reagent  has  been  used,  this 
will  be  of  a  dark-red  color.  The  liquid  is  now  filtered,  and 
50  mils  of  the  filtrate  (representing  one-half  of  the  alkaloid) 
is  treated  with  a  10  per  cent  solution  of  sodium  thiosulphate 
added,  drop  by  drop,  until  the  color  of  the  free  iodin  dis- 
appears. This  discolorization  is  not  needed  if  Mayer's  reagent 
has  been  used.  A  few  drops  of  the  indicator  phenolphthalein 
are  now  introduced,  and  the  excess  of  acid  estimated  by  reti- 

N  . 

tration  with  —  potassium  hydroxid  solution.     This,  deducted 

from   one-half  of  the  volume  of  the   standard  acid  solution 

.  N  . 

employed,   indicates  the  number  of  mils  of  —  hydrochloric 

acid  solution,  which  combined  with  the  alkaloid  in  50  mils 

of  the  solution.     This  number,  multiplied  by  two  and  then 

by  the  factor  for  the  alkaloid  present,  gives  the  total  quantit)- 

of  alkaloid. 

Example.    An  alkaloidal  residue  consisting  of  morphine 

N 
was  dissolved  in  30  mils  of  —  HCl  V.S.    Then  Wagner's  reagent 

was  added  in  excess  as  described,  and  the  mixture  made  up 


258      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

to  loo  mils  with  water.     Fifty  mils  of  this  were  filtered  off, 

N 
decolorized  as  directed,  and  titrated  with  —  KOH  V.S.     Ten 

20 

mils  were  required,  which  corresponds  to  20  mils  for  the  entire 

N 
quantity.     Then   20  mils  deducted   from   the  30  mils  of  — 

HCl  V.S.  added,  leaves  10  mils,  the  quantity  required  to  neu- 

N 
tralize  the  m^orphine.     The   —  factor  for  morphine    (0.0137 

gm.,   Gordin),  multiplied  by  10  =  0.137  gm.,  the  quantity  oi 

alkaloid  in  the  residue  examined. 

N 
The  —  factor  for  morphine  here  given  is  somewhat  lower 

than  the  theoretical  equivalent.     It  was  ascertained  by  exper- 
ment  with  a  sample  of  the  anhydrous  alkaloid. 

The  following  factors  (by  Gordin)  were  obtained  by  com- 
paring their  molecular  weights  with  that  of  morphine,  which 
factor  was  determined  by  experiment: 

N 
Morphine 0-0137  gm.  =  i  mil  —  HCl  V.S. 

Hydrastine 0.0184 

Strychnine 0.0160 

Caffeine,  cryst 0.0102 

Cocaine 0.0146 

Atropine 0.0139    "   = 


CHAPTER  XII 

VOLUMETRIC    ASSAYING    OF    VEGETABLE    DRUGS 
AND  THEIR  PREPARATIONS 

EXTRACTION   OF   THE   ALKALOIDS 

Selection  of  the  Sample.  Care  must  be  taken  to  secure 
a  fairly  representative  sample. 

If  the  drug  is  in  small  pieces  or  consists  of  seeds  or  leaves, 
mix  it  well,  take  a  portion,  pulverize  it,  and  of  the  powder 
take  a  sufficient  quantity  for  the  assay.  If  the  drug  be  in 
large  lumps,  which  vary  in  quality,  select  a  few  representative 
lumps  and  cut  from  each  a  fairly  representative  section;  pul- 
verize these,  mix  well,  and  weigh  off  a  sufficient  quantity  for 
the  assay. 

If  drying  is  necessary,  the  loss  of  weight  in  drying  must 
be  made  note  of. 

The  Exhaustion  of  the  Drug  is  usually  effected  by  macer- 
ation in  a  suitable  menstruum,  although  percolation,  boiling, 
and  hot  repercolation  must  be  employed  in  some  cases. 

The  Choice  of  Solvent  depends  upon  the  nature  of  the 
drug.  Water  dissolves,  besides  the  alkaloids,  so  much  inert 
matter  that  the  subsequent  steps  in  the  assay  are  liable  to 
be  interfered  with.  Alcohol  dissolves  too  much  of  the  resinous 
matter,  and  besides  does  not  penetrate  the  drug  very  well. 
Acidulated  water  has  been  much  used,  but  chloroform  and 
ether,  separately  and  in  various  combinations,  are  now  most 
generally  employed  for  exhausting  drugs  in  conjunction  with 

259 


260      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

alcohol  and  ammonia.  Petroleum  benzin  has  of  late  been 
recommended. 

Prollius'  fluid,  or  some  modification  of  it,  is  very  satis- 
factory. 

Prollius'  Fluid  consists  of  ether  325  mils,  alcohol  25  mils, 
and  concentrated  ammonia-water  10  mils. 

Modified  Prollius'  Fluid  consists  of  ether  250  mils,  chloro- 
form 80  to  100  mils,  alcohol  25  mils,  concentrated  ammonia- 
water  10  mils. 

The  Separation  of  the  Alkaloids  and  the  Use  of  Immiscible 
Solvents.  "  The  Shaking-Out  "  Process.  Most  alkaloids  are 
insoluble  in  water  but  are  readily  soluble  in  certain  i'  volatile 
solvents  "  such  as  alcohol,  ether,  chloroform,  amyl  alcohol, 
benzene,  petroleum  benzin,  acetic  ether,  etc.,  but  not  in 
carbon  tetrachlorid.  Those  alkaloidal  solvents  which  do  not 
mix  readily  with  water  are  known  as  "  the  immiscible  sol- 
vents." Salts  of  alkaloids  on  the  other  hand  are  as  a  rule 
soluble  in  water,  but  practically  insoluble  in  the  immiscible 
solvents.  This  property  of  the  alkaloids  makes  it  possible 
to  separate  them  from  their  natural  sources. 

The  alkaloid  in  solution  in  an  immiscible  solvent  may  be 
separated  by  "  shaking  out  "  with  a  dilute  acid,  the  alkaloid 
is  thereby  converted-  into  an  alkaloidal  salt  which  readily 
dissolves  in  the  aqueous  acid  solution.  Conversely,  an  aqueous 
solution  of  an  alkaloidal  salt  when  treated  with  an  alkali  to 
set  free  the  alkaloid  and  shaken  with  an  immiscible  solvent 
gives  up  its  alkaloid  in  a  pure  state  to  the  latter.  The  process 
of  assay  by  this  "  shaking-out  "  process  is  carried  out  by 
treating  the  liquid  extracts  (after  having  been  freed  from 
alcohol)  with  an  immiscible  solvent  and  a  slight  excess  of 
an  alkaH.  The  alkaloid  is  thus  dissolved  in  the  immiscible 
solvent,  which  is  then  separated  from  the  aqueous  liquid, 
and  transferred  to  another  vessel  in  which  it  is  shaken  with  an 


VOLUMETRIC  ASSAYING  OF  VEGETABLE  DRUGS     261 


excess  of  acid  largely  diluted  with  water.  The  acid  combines 
with  the  alkaloid  and  forms  an  alkaloidal  salt,  which  leaves 
the  immiscible  solvent,  and  enters  the  aqueous  solution.  If 
the  alkaloidal  solution  so  obtained  is  still  colored,  the  shaking 
out  is  repeated  until  a  pure  alkaloidal  solution  in  the  immis- 
cible solvent  is  obtained.  The  ap- 
paratus used  in  this  shaking-out 
operation  is  known  as  a  separator. 
See  Fig.  54. 

Separators  are  conical  or  pear- 
shaped;  the  neck  is  provided  with 
a  well-ground  glass  stopper  and  the 
oudet  tube  or  stem  at  the  bottom 
with  an  accurately-fitting  glass  stop- 
cock. The  solvents  usually  used 
in  alkaloidal  drug  assaying  _  are 
alcohol,  chloroform,  ether  and  vari- 
ous mixtures  of  these  containing  at 
least  75  per  cent  of  ether.  When 
chloroform  is  used  it  will  collect  at 
the  bottom  of  the  separator,  and  can 
be  easily  drawn  off;  but  if  ether,  or  ether-chloroform  mixture  is 
used,  it  will  form  the  upper  layer  in  the  separator,  and  may  be 
syphoned  off,  or  the  lower  aqueous  layer  drawn  off,  and  the 
ethereal  layer  then  transferred  to  another  separator.  Violent 
shaking  of  the  contents  of  the  separator  is  to  be  avoided, 
gentle  shaking  or  rotating  is  quite  sufficient  to  bring  about  the 
desired  transfer  of  alkaloid.  Excessive  shaking  will  cause 
the  formation  of  an  emulsion  of  the  water  and  solvent,  which 
is  often  hard  to  break  up. 

The  final  operation  should  always  be  the  collection  of  the 
free  alkaloid  by  means  of  a  portion  of  the  immiscible  solvent. 
This  solution  is  drawn  off  into  a  beaker,  the  solvent  evap- 


Squibb's  Pattern 

Fig.  54. 


262       THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

orated  over  a  water-bath,  using  gentle  heat,  and  the  dry- 
residue  in  the  beaker  dissolved  in  a  measured  quantity  of 
standard  acid  V.S.  The  excess  of  acid  V.S.  is  then  titrated 
with  a  standard  alkali  V.S.  in  the  presence  of  a  suitable 
indicator. 

In  connection  with  this,  attention  is  called  to  the  fact  that 
some  of  the  most  important  methods  of  isolating  alkaloids 
in  drug  assays  and  in  toxicological  investigations  depend  upon 
the  solubility  of  the  free  alkaloids  in  ether,  chloroform,  benzene, 
etc.,  and  the  relative  insolubility  of  alkaloidal  salts  in  the 
same  solvents.  There  are,  however,  a  number  of  exceptions 
to  this  rule,  and  special  attention  is  called  to  the  fact  that  in 
many  cases  alkaloids  pass  from  decidedly  acid  aqueous  solu- 
tions (in  which  they  certainly  occur  as  alkaloidal  salts)  into 
chloroform  and  ether,  in  the  shaking-out  methods.  Wliile 
the  amount  of  alkaloid  so  dissolved  by  these  solvents  is  never 
very  great,  still  the  quantity  is  an  appreciable  one.  This 
behavior  occurs  in  the  case  of  caffeine,  colchicine,  and  narco- 
tine;  and  under  certain  conditions  also  in  the  case  of  strychnine, 
atropine,  veratrine  and  other  bases.  This  transfer  of  alkaloid 
to  chloroform  occurs  more  particularly  in  the  case  of  alka- 
loids of  weak  basic  character,  and  when  the  solution  is  neutral 
or  only  feebly  acid,  or  when  the  alkaloid  is  in  combination 
with  a  comparatively  weak  acid,  as  citric,  tartaric,  etc.  Fur- 
thermore, some  alkaloidal  salts,  notably  those  of  weak  bases, 
are  transferred  as  such  to  chloroform,  especially  the  salts  of 
hydrochloric  acid,  hydrobromic  acid,  and  nitric  acid.  In  the 
case,  however,  of  the  sulphates,  phosphates,  tartrates,  and 
citrates  of  strongly  basic  alkaloids,  no  -transfer  occurs,  or  at 
most,  only  minute  quantities  of  the  alkaloids  pass  over.  In 
order  to  prevent  a  transfer  of  alkaloid  or  alkaloidal  salt  out 
of  an  aqueous  solution  to  an  immiscible  solvent,  the  use  of 
sulphuric  acid  is  to  be  recommended,  and  in  all  toxicological 


VOLUMETRIC  ASSAYING  OF  VEGETABLE  DRUGS     263 

investigations  due  regard  should  be  paid  to  the  above  named 
conditions.  For  more  details,  see  thejpaper  by  Edward  Schaer, 
Proc.  A.  Ph.  A.,  1906,  425.  ^ 

GENERAL  METHODS  OF  ASSAYING  DRUGS 

No  rule  can  be  formulated  as  to  the  method  of  extraction, 
or  the  solvent  to  be  employed,  which  can  be  applied  to  all  * 
drugs.  Each  must  be  dealt  with  in  accordance  with  the 
properties  of  the  contained  alkaloids  and  their  state  of  com- 
bination. Several  methods,  however,  are  in  use*which  may 
be  applied  to  a  large  number  of  different  drugs,  and  with 
sHght  special  modifications  to  many  more. 

Kebler's  Modification  of  the  Keller  Method  *  Treat  10 
gms.  of  the  dry  powdered  drug  in  a  250-mil  flask  with  25  gms. 
of  chloroform  and  75  gms.  of  ether;  stopper  the  flask  securely, 
agitate  well  for  a  few  minutes  and  add  10  gms.  of  ammonia- 
water  and  shake  frequently  during  one  hour.  Then  on  adding 
5  gms.  more  of  the  ammonia-water  and  shaking,  the  suspended 
powder  agglutinates  into  a  lump,  leaving  the  solution  clear 
after  a  few  minutes'  standing.     Then  proceed  by  A  or  B.  ^ 

A.  When  the  mixture  has  completely  separated,  50  gms. 
(representing  5  gms.  of  the  drug)  are  poured  off  into  a  beaker 
and  heated  on  a  water-bath  until  the  solvent  is  evaporated. 
Ten  mils  of  ether  are  then  added  and  again  evaporated.  The 
varnish- like  residue  is  then  dissolved  in  15  mils  of  warm  alcohol 
and  water  added  to  slight  permanent  turbidity,  then  the 
indicator  is  added,  followed  by  an  excess  of  standard  acid 
solution,  and  the  mixture  retitrated  with  standard  alkali. 

B.  When  the  mixture  has  completely  separated,  pour  off 
50  gms.  into  a  separatory  funnel,  and  add  20  mils  of  acidulated 
water,   agitate,   and   when   the   liquids   have    separated   draw 

*  J.  A.  c.  S.,  XVII,  828. 


264      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

off  the  aqueous  solution  into  a  second  separ^ory  funnel. 
Repeat  this  operation  with  two  more  portions  of  15  mils  of 
acidulated  water.  Now^nder  the  contents  of  the  separatory 
funnel  alkaline  by  ad^J^ng  ammonia- water.  This  liberates  the 
alkaloids,  which  are  theij  separated  by  treatment  with  a  mix- 
ture of  chloroform,  three  parts  (by  volume),  and  ether,  one 
part,  using  three  successive  portions,  first  20  mils,  then  twice 
i5(fcnils. 

The  chloroform-ether  solution  is  heated  on  a  water-bath 
until  the  solvent  is  evaporated,  and  then  the  varnish-like 
residue  treated  twice  with  8  mils  of  ether  and  again  evaporated. 

The  residue  is  then  dissolved  in  15  mils  of  alcohol,  water 
added  to  slight  permanent  turbidity,  and  then  the  indicator. 
Titrate  in  usual  way  with  decinormal  acid  and  centinormal 
alkaH. 

A  serious  objection  to  this  method  lies  in  the  taking  of  a 
so-called  aliquot  part:  First,  because  of  the  well-known  solu- 
bility of  ether  in  water,  and  conversely  of  water  in  ether,  as 
a  result  of  which  the  volume  of  the  ethereal  stratum  is  materially 
changed.  Furthermore,  commercial  ether  contains  variable 
quantities  of  alcohol,  hence  the  change  in  volume  will  not 
always  be  the  same. 

Another  source  of  error  in  the  aliquot  part  is  found  in 
the  volatile  nature  of  the  solvents  used.  In  warm  weather 
it  is  impossible  to  avoid  loss  by  volatilization,  hence  the  aliquot 
part  taken  is  too  large. 

W.  A.  Puckner  *  has  described  a  modification  of  the 
Keller  method  which  avoids  the  use  of  the  aliquot  part.  He 
uses  only  one-half  of  the  ethereal  solvent  for  the  maceration, 
and  after  the  usual  maceration  transfers  the  drug  to  a  small 
percolator  in  which,  after  the  ethereal  solution  has  been  well 

*  Ph.  Rev.  XVI,    180,  and  XX,  457. 


m 


VOLUMETRIC  ASSAYING  OF  VEGETABLE  DRUGS     265 

drained  off,  the  marc  is  percolated  with  the  same  menstruum 
to  complete  exhaustion.  The  quantity  of  ethereal  solvent 
required  is  not  materially  greater  than  in  the  Keller  method, 
while  the  quantity  of  alkaloid  obtained  for  weighing  or  titrating 
is  larger  because  it  represents  the  whole  of  the  sample  taken 
for  the  assay.  In  the  case  of  drugs  containing  a  very  small 
proportion  of  alkaloid  this  is  an  important  advantage. 

The  objection  to  this  plan  is  that  the  transfer  of  the  mass 
from  the  flask  in  which  the  maceration  has  been  conducted 
to  a  suitable  percolator,  which  should  not  be  more  than  3  cm. 
in  diameter,  requires  very  "dextrous  manipulation,  or  it  will 
be  attended  with  loss  of  alkaloid. 

Assay  of  Galenical  Preparations. 

J.  U.  Lloyd's  Method.  One  gm.  of  a  solid  extract  which 
has  been  dissolved  lq  5  to  8  mils  of  an  alcoholic  mefistruum  or 
a  corresponding  volume  of  the  tincture  evaporated  to  this 
bulk,  or  5  mils  of  the  fluid  extract  in  a  flat-bottomed  porcelain 
mortar,  are  mixed  with  2  mils  of  a  solution  of  perchloride  of 
iron.  To  this  is  added  sodium  bicarbonate  with  constant 
trituration  until  a  stiff  magma  *  results.  Extract  this  magma 
by  repeated  trituration  with  chloroform,  using  first  20  mils 
and  then  three  portions  of  10  mils  each,  decanting  them  severally 
by  means  of  a-  guiding-rod,  being  careful  that  no  suspended 
portions  of  the  magma  are  drawn  off.  In  order  to  make  sure 
that  all  of  the  alkaloid  has  been  extracted,  add  5  mils  more  of 
chloroform,  draw  it  off,  evaporate  on  a  watch-glass,  dissolve 

*  The  ferric  hydroxid  which  is  produced  in  the  above  process  serves  to 
attract  most  of  the  tannates,  gums,  vegetable  acids,  and  coloring  matters, 
while  the  excess  of  sodium  bicarbonate  liberates  the  alkaloids,  which  are  then 
dissolved  by  the  chloroform.  If  the  fluid  extract  is  strongly  alcoholic  the 
chloroform  will  not  separate  easily,  in  which  case  the  addition  of  a  few  mils 
of  water  containing  a  very  little  glucose  will  cause  a  sharp  separation. 


I 

266       THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

residue  in  dilute  sulphuric  acid,  and  test  for  alkaloids  by 
Wagner's  or  Mayer's  reagent. 

The  mixed  chloroformic  extracts  are  collected  and  may  be 
estimated  volumetrically  as  follows: 

Method  A.  To  be  used  if  the  chloroformic  extract  is  not 
colored. 

The  chloroformic  solution  is  evaporated  to  dryness  in  a 

flask  placed  on.  a  water-bath.     To  this  residue  is  added  an 

N 
accurately  measured  excess  of  —  sulphuric  V.S.  and  the  solution 

diluted  with  a  little  water,  the  indicator  added  and  the  excess 

N 
of  standard  acid  solution  estimated  by  titrating  with  —  potas- 

N 
sium  hydroxid  V.S.     The  number  of  mils  of  the  —  alkali  V.S. 

N 
used,  divided  by  2  and  subtracted  from  the  volume  of  —  acid 

•V.S.  originally  added,  will  give  the  number  of  mils  of  the 

latter  required  for  the  alkaloid.     This  number,  multiplied  by 

the  proper  factor,  will  give  the  total  alkaloid  present  in  the 

fluid  extract. 

Example.    The  chloroformic  residue  obtained  from  5  mils 

of  a  fluid  extract  of  hyoscyamus  was  dissolved  in  12  mils  of 

N  N 

—  acid  V.S.,  the  solution  titrated  with  —  alkali  V.S.     20.6 

25  50 

mils  of  the  latter  were  used. 

"     N  .  N 

20.6  mils  of  —  V.S.  is  the  equivalent  of  10.3  mils  of  — 

N 
V.S.     10.^  mils  subtracted  from  12  mils,  the  amount  of  —  acid 

o  25 

'^'  .  N       . 

originally  added,  leaves  1.7  mils,  the  quantity  of  —  acid  V.S. 

which  was  required  for  the  alkaloid.     This,  multiplied  by  the 


VOLUMETRIC  ASSAYING  OF  VEGETABLE  DRUGS    267 

N 

—  factor  for  total  alkaloids  of  hyoscyamus,  0.01157  gm.,  gives 

tfee  quantity  of  alkaloids  present  in  the  5  mils,  which  quantity 
multiplied  by  20  gives  the  per  cent : 

1.7X0.01157  =  0.019669  gm.X2o  =  o.39338  per  cent. 

Method  B.  This  method  may  be  employed  if  the  chloro- 
formic  extract  is  highly  colored,  the  indicator  fluorescein  being 
used. 

The  residue  from  the  evaporation  of  chloroform  is  dis- 
solved in  10  mils  of  acid-free  alcohol ;  then  water  is  added  to 

N     . 
slight  turbidity,  followed  by  a  measured  excess  of  —  acid  V.S.; 

N  ^^ 

then  the  titration  is  completed  with  —  alkali  V.S.     The  first 

appearance  of  fluorescence  marks  the  completion  of  the  reaction. 
This  is  best  observed  by  holding  the  flask  over  a  dark  surface 
and  viewing  by  reflected  light. 

Method  C.  This  is  to  be  used  in  the  case  of  highly  colored 
extracts.  It  consists  in  removing  the  alkaloid  in  a  pure  state 
by  shaking  out  in  a  separating  funnel  with  immiscible  sol- 
vents. The  chloroformic  extract  is  shaken  out  with  several 
portions  of  acidulated  water;  this  removes  the  alkaloid,  leaving 
resins,  fats,  coloring  matters,  etc.,  in  the  chloroform.  The 
acid  alkaloidal  solution  is  then  treated  with  ether  in  a  second 
separator,  ammonia  added  to  alkaline  reaction,  a.nd  the  alka- 
loid thus  liberated  by  ammonia  dissolves  in  the  ether,  from 
which  it  is  obtained  by  evaporation  and  estimated  acidimetrically. 

Another  Method.  Add  ammonia  and  shake  out  directly 
with  an  immiscible  solvent.  Vl^ash  the  alkaloid  out  of  this 
with  acidulated  water  and  again  shake  out  with  a  suitable 
immiscible  solvent. 

J.  Katz's  Method  (Arch.  d.  Pharm.,  1898,  i;  Am.  Dr.y 
1898,  281) .     This  method  has  the  advantage  of  all  other  methods 


268      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

in  that  it  enables  one  to  rapidly  and  accurately  estimate  the 
alkaloids  in  a  preparation  without  the  necessity  of  applying 
heat  for  any  purpose  whatever  during  the  process.  The  assay 
may  be  made  in  from  one  to  three  hours.  Twenty-five  mils  of 
the  tincture  of  an  alcoholic  strength  of  about  45  per  cent  are 
placed  in  a  separatory  funnel,  i  mil  of  a  33  per  cent  solution 
of  soda  added,  and  the  mixture  shaken  for  five  minutes  with 
50  mils  of  ether,  and  set  aside  until  the  liquids  have  com- 
pletely separated  into  two  layers.  The  lower  dark-colored 
aqueous  layer  is  drawn  off  into  a  beaker.  The  ethereal  layer 
which,  besides  the  alkaloid,  has  taken  up  most  of  the  alcohol 
and  some  coloring  matter,  is  shaken  up  with  3  mils  of  water 
which,  after  separating,  is  drawn  off  and  added  to  the  first 
aqueous  liquid.  The  ethereal  liquid  is  then  poured  into  a 
suitable  flask,  while  the  combined  aqueous  liquid  is  further 
treated  with  two  portions  of  ether  (25  mils  each)  the  ether  to 
contain  about  10  per  cent  of  alcohol.  These  ethereal  extractions 
are  washed  each  with  1.5  mils  of  water;  the  first  extraction 
after  washing  may  be  added  to  the  original  ethereal  solution, 
while  the  second  is  reserved  and  later  on  employed  for  washing 
the  flask.  The  ethereal  solution,  which  contains  still  some 
traces  of  aqueous  fluid  containing  alkali,  is  dehydrated  by 
treatment  with  2  or  3  gms.  of  exsiccated  calcium  sulphate, 
and  finally  filtered  into  a  glass-stoppered  flask  containing  50 

mils  of  water. 

N 
Titration  by  means  of  acid  V.S.  is  employed,  using 

alcholic  solution  of  iodeosin  (1-250)  as  indicator. 

The  Method  as  above  described  is  obviously  applicable 
only  to  such  alkaloids  as  are  readily  soluble  in  ether.  If  an 
estimation  of  alkaloids  insoluble  or  only  slightly  soluble  in 
ether,  but  soluble  in  chloroform,  is  to  be  made,  the  method 
is  modified  as  follows: 


VOLUMETRIC  ASSAYING  OF  VEGETABLE  DRUGS    269 

Twenty-five  mils  of  the  tincture  of  45  per  cent  alcoholic 
strength  are  vigorously  shaken  for  five  minutes  with  30  mils 
of  a  mixture  of  i  part  of  chloroform  and  2  parts  of  ether.  The 
solution  so  obtained  is  washed  with  3  mils  of  a  30  per  cent 
solution  of  sodium  chlorid.  This  operation  is  repeated  twice,, 
using  each  time  15  mils  of  the  ether-chloroform  mixture  and 
washing  with  1.5  mils  of  sodium  chlorid  solution. 

If  the  Separation  of  the  aqueous  and  ethereal  liquids  is  not 
distinct  an  additional  2  or  3  gms.  of  sodium  chlorid  may  be 
used.  This  prevents  the  emulsification,  which,  if  pure  water 
were  employed,  would  occur. 

If  the  Tincture  to  be  assayed  contains  more  than  45  per 
cent  of  alcohol  it  is  necessary  to  add  water  to  reduce  it  to  40 
or  50  per  cent. 

Tinctures  containing  Chlorophyll  or  fat  or  fatty  acids  must 
first  be  deprived  of  these  constituents,  as  these  substances, 
possessing  acid  properties  inferior  to  that  of  iodeosin,  will  act 
the  part  of  an  alkali  toward  it  and  thus  be  recorded  as  alkaloid. 

To  remove  the  Chlorophyll  and  fatty  acids,  acidulate  a 
mixture  of  equal  parts  of  the  tincture  and  water  with  a  few 
drops  of  sulphuric  acid,  shake  with  talcum  during  several 
hours,  and,  after  subsidence  of  the  latter,  filter.  Of  this 
filtrate  25  gms.  (not  mils,  on  account  of  the  admixture  of  alco- 
hol and  water  causing  change  of  volume)  are  taken  and  the 
alkaloid  estimated  in  the  manner  already  described,  after 
first  removing,  if  necessarj,  the  last  traces  of  fat  by  a  single 
shaking  of  the  acid  solution  with  petroleum  ether. 

For  the  Assay  of  extracts  i  to  1.5  gms.  are  dissolved  in 
from  40  to  50  mils  of  45  per  cent  alcohol  to  make  a  solution 
containing  less  than  3  per  cent  extractive.  For  the  assay  of 
fluid  extracts  10  mils  are  taken. 

For  details  of  special  assays,  see  the  author's  Manual  of 
Volumetric  Analysis. 


270       THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

The  Influence  of  the  Presence  of  Certain  Volatile  Solvents 
upon  the  accuracy  of  alkaloidal  titrations  must  be  borne  in 
mind.  Alcohol  in  may  instances  influences  the  color  changes 
of  the  indicators  to  the  extent  of  rendering  them  indistinct 
or  entirely  unreliable;  while  ether  or  chloroform  materially 
diminishes  the  sensitiveness  of  many  of  our  most  valued 
indicators,  among  them  phenolphthalein,  rosalic  acid,  congo- 
red,  and  luteol.  On  the  other  hand,  fluorescein  and  gallein 
may  be  mentioned  as  being  more  sensitive  in  the  presence  of 
alcohol  or  even  of  ether.  It  is  advisable,  therefore,  in  most 
cases,  to  perform  the  titration  without  the  presence  of  the 
above-named  solvents. 

Indicators  Vary  in  their  Degree  of  Sensitivenness  to  the 
same  Alkaloid,  hence  the  choice  of  an  indicator  in  a  particular 
case  is  a  matter  of  importance.  The  following  table,  based 
upon  that  of  Kippenberger,  will  serve  as  an  aid  in  the  selection. 
The  smallest  quantity  which  will  procure  a  distinct  tint  should 
be  taken. 

Atropine.  Lacmold,  Fluorescein,  lodeosin,  Haematoxylin. 

Brucine.  Cochineal,  lodeosin,  Haematoxylin. 

Cocaine.  Lacmoid,  Fluorescein,  Cochineal,  Haematoxylin. 

Coniine.  Cochineal,  Lacmoid,  lodeosin,  Haematoxylin. 

Codeine.  lodeosin,  Lacmoid,  Haematoxylin. 

Emetine.  lodeosin.  Cochineal. 

Morphine.  Hcematoxylin,  Cochineal,  Lacmoid. 

Quinine.  Hcematoxylin,  Azolitmin,  Fluorescein. 

Strychnine.  Azolitmin,  lodeosin,  Haematoxylin. 

Sparteine.  Hcematoxylin,  Azolitmin. 


CHAPTER  XIII 

ESTIMATIONS  INVOLVING   USE   OF  DECINORMAL 
BROMIN  V.S. 

Preparation  of  Decinormal  Bromin  V.S.  (Koppeschaar's 
Solution),  Br=  79.92;    7.992  gm.  in  i  liter. 

KBr      =119.02         NaBr      =102.92 
KBr03  =  167.02         NaBr03  =  150.92 

This  solution  does  not  contain  free  bromin,  but  it  contains 
two  salts,  a  bromid  and  a  bromate,  which,  when  treated  with 
hydrochloric  acid,  liberate  a  definite  quantity  of  bromin. 

It  is  made  as  follows:  Dissolve  3  gms.  of  sodium  bromate 
and  50  gms.  of  sodium  bromid  (or  3.2  gms.  of  potassium 
bromate  and  50  gms.  of  potassium  bromid)  in  sufficient  water 
to  make  900  mils. 

Transfer  20  mils  of  this  solution  by  means  of  a  pipette  into 
a  bottle  having  a  capacity  of  about  250  mils,  provided  with  a 
glass  stopper;  add  75  mils  of  water,  then  5  mils  of  pure  hydro- 
chloric acid,  and  immediately  insert  the  stopper. 

Shake  the  bottle  a  few  times,  then  remove  the  stopper 
just  sufficiently  to  quickly  introduce  5  mils  of  potassium  iodid 
solution  (1-5),  taking  care  that  no  bromin  vapor  escape,  and 
immediately  stopper  the  bottle. 

Agitate  the  bottle  thoroughly,  remove  the  stopper  and 
rinse  it  and  the  neck  of  the  bottle  with  a  little  water  so  that 
the  washings  flow  into  the  bottle,  then  add  from  a  burette 
decinormal  sodium  thiosulphate  until  the  color  of  the  free 
iodin  is  nearly  all  discharged,  then  add  a  few  drops  of  starch 

271 


r 


272       THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

N 
solution,  and  continue  the  titration  with  —  thiosulphate  V.S. 

until  the  blue  color  disappears. 

N 
Note  the   number  of  mils  of  the  —  sodium  thiosulphate 

thus  used,  and  dilute  the  bromin  solution  so  that  equal  volumes 

N 
of  it  and  the  —  sodium  thiosulphate  will  exactly  correspond 

to  each  other  under  the  above-mentioned  conditions. 

Example.    Assuming  that  the  20  mils  of  bromin  solution 

N 
required  25.2  mils  of  the  —  thiosulphate  to  completely  absorb 

the  iodin,  the  bromin  solution  must  be  diluted  in  the  pro- 
portion of  20  to  25.2;  that  is,  each  20  mils  must  be- diluted  to 
make  25.2  mils. 

Thus  if  850  mils  are  left,  they  must  be  diluted  to  make 
107 1  mils,  and  the  solution  is  decinormal, 

A  new  trial  should  always  be  made  after  diluting,  and 
the  bromin  solution  should  correspond,  volume  for  volume, 
with  the  decinormal  sodium  thiosulphate. 

The  first  step  in  the  preparation  of  this  solution  is  to 
dissolve  the  salts;  then  iiydrochloric  acid  is  added,  which 
liberates  a  definite  quantity  of  bromin,  as  the  equation  illus- 
trates : 

5NaBr  +  NaBrOs  +  6HC1  =  6NaCl  -f  ^Brs  +  3H2O. 

The  stopper  should  be  inserted  iuto  the  bottle  as  soon 
as  the  hydrochloric  acid  has  been  added,  in  order  that  no 
bromin  vapor  escape,  and  the  bottle  rotated  so  as  to  mix  the 
acid  thoroughly  with  the  liquid. 

The  next  step  is  to  determine  the  quantity  of  bromin 
which  a  definite  volume  of  solution  will  liberate.  The  bromin 
solution  should  be  of  such  strength  that  1000  mils  of  it  will 


ESTIMATION  OF  PHENOL  273 

contain  7.976  gms.  of  available  bromin.  Bromin,  like  chlorin, 
liberates  iodin  from  potassium  iodid,  and  is  estimated  in 
the  same  manner. 

One  atomic  weight  of  iodin  is  liberated  by  one  atomic 
weight  of  bromin: 

Br2  +  2KI  =  2KBr  +  l2 

Thus  by  determining  the  quantity  of  iodin  liberated  the 
quantity  of  bromin  is  found. 

N 
The  iodin  is  determined  by  the  —  sodium  thiosulphate, 

one  liter  of  which  represents  12.692  gms.  of  iodin,  which  is 
equivalent  to  7.992  gms.  of  bromin,  as  shown  by  the  following 
equation : 


(Bra)       =       I2     +     2(Na2S203  +  5H20) 

20)159.84      20)253.84  2o)4q6.44  j^ 

7.992  gms.    12.692  gms.         24.822  gms,  or  1000  mils  —  V.S. 

10 

=  2NaI  +  Na2S406  +  10H2O. 

The  Assay  of  Phenol.  Dissolve  i  gm.  of  the  sample  in 
sufficient  water  to  make  500  mils  of  solution  at  the  standard 
temperature.  Twenty  mils  of  this  solution  containing  0.04  gm. 
of  the  sample  are  transferred  to  a  glass-stoppered  bottle,  having 
a  capacity  of  about  200  mils. 

To  this,  30  mils  of  decinormal  bromin,  followed  by  5  mils 
of  hydrochloric  acid,  are  added,  and  the  bottle  immediately 
stoppered,  and  shaken  repeatedly  during  half  an  hour. 

Then  the  stopper  is  removed  just  sufficiently  to  introduce 
5  mils  of  a  20  per  cent  aqueous  solution  of  potassium  iodid, 
being  careful  that  no  bromin  escape. 

The  bottle  is  then  thoroughly  shaken  and  the  neck  rinsed 
with  a  little  water,  the  washings  being  allowed  to  flow  into  the 
bottle,  then  i  mil  of  chloroform  is  added,  the  mixture  thoroughly 


274     THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

shaken,  and  titration  with  decinormal  sodium  thiosulphate 
V.S.  begun,  starch  being  used  as  indicator,  until  the  blue  color 
is  just  discharged. 

The  precipitated  tribromphenol  interferes  somewhat  with 
the  end-reaction  when  starch  is  used,  and  frequently  with 
old  phenol  solutions  *  the  precipitate  possesses  a  bluish  color 
which  is  not  removed  by  an  excess  of  sodium  thiosulphate  and 
which  makes  the  end-reaction  difficult.  This  difficulty  is  over- 
come by  the  use  of  a  small  quantity  of  chloroform  which  dissolves 
the  tribromphenol  and  admits  of  a  very  sharp  end-reaction. 

When  chloroform  is  alone  used,  the  end-reaction  is  very 
clearly  defined,  and  is  known  by  a  colorless  aqueous  solution 
and  the  chloroform  being  free  from  any  tinge  of  pink,  due  to 
traces  of  iodin. 

N 
Note  the  number  of  mils  of  —  thiosulphate  used;   deduct 

N 
this  number  from  30  mils  (the  quantity  of  —  bromin  originally 

N 
added),  and  the  quantity  of  —  bromin  which  went  into  com- 
bination with  the  phenol  is  obtained. 

N 
Each  mil  of  —  bromm  represents  0.001556  gm.  of  pure 

phenol. 

N 
Example.    Assuming  that  6  mils  of  —  sodium  thiosulphate 

were  required  to  discharge  the  color  of  the  starch  iodid,  this 
deducted  from  30  mils  leaves  24  mils,  the  quantity  which 
combined  with  the  phenol. 

0.001566X24=0.037584  gm. 
0.037584X100 


0.04 


=  93.96  per  cent  of  pure  phenol. 


*  F.  X.  Moerk,  A.  J.  Ph.,  1904,  475. 


ESTIMATIONS  OF  DECINORMAL  BROMIN  V.S.      275 

The  above  method  originated  with  Koppeschaar,  and  is 
the  only  volumetric  method  by  which  accurate  results  may 
be  obtained. 

It  is  based  upon  the  fact  that  bromin  reacts  with  phenol, 
producing  an  insoluble  precipitate  of  tribromphenol. 

The  titration  is  not  made  directly;  but  the  phenol  solution 
is  treated  with  an  excess  of  standard  bromin  solution  in  the 
presence  of  some  hydrochloric  acid.  The  hydrochloric  acid 
liberates  the  bromin,  and  the  freed  bromin  reacts  with  the 
phenol,  as  shown  by  the  equations: 

{a)         sNaBr  +  NaBrOa  +  6HC1  =  6NaCl  +  3H2O  +  sBra 

(6)        CeHsOH     +     3Br2   =   CeHsBraOH   +  aHBr. 

6)94  6)479.92 

10)  IS -66  10)79-92  j^ 

1.566  gms.  7-992  gms.  or  1000  mils  —  bromin  V.S. 

N 
Thus  each  mil  of  the  —  bromin  represents  0.001566  gm. 

of  pure  phenol. 

The   bromin   solution   which   was   added   in   excess,    and 

the  liberated  bromin  of  which  is  not  fixed  by  phenol,  is  then 

N 
found  by  residual  titration  with  —    sodium   thiosulphate   after 

the  addition  of  some  potassium  iodid. 

The  decinormal  bromin  solution  and  the  decinormal  sodium 
thiosulphate  solution  being  equivalent,  each  mil  of  the  latter 
consumed  represents  one  mil  of  the  former.  Then  by  sub- 
tracting the  number  of  mils  of  the  sodium  thiosulphate  solu- 
tion from  the  number  of  mils  of  bromin  solution  originally 
added,  the  quantity  of  the  latter  which  was  actually  consumed 
by  the  phenol  present  is  found.  This  number,  when  igtil-^ 
tiplied  by  the  factor  for  phenol,  then  gives  the  quantity  of 
pure^phenol  present.  *^' 


276      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

The  hydrochloric  acid  used  in  the  above  estimation  must 
contain  no  free  chlorin.  The  potassium  iodid  must  be  free 
from  iodate.  The  starch  T.S.  should  not  be  added  until 
most  of  the  free  iodin  has  been  taken  up,  and  the  color  of 
the  solution  has  diminished  to  light  yellow. 

The  carbolic  acid  should  be  diluted  with  water  before 
titration,  and  should  never  be  stronger  than  o.i  gm.  in  25  mils. 

Resorcinol,  C6H4(OH)2  i  :  3  (110.05).  Dissolve  about  1.5 
gm.,  accurately  weighed,  in  sufficient  distilled  water  to  measure 
500  mils.  Transfer  25  mils  of  this  solution  (representing  0.075 
gm.)  to  a  500-mil  glass-stoppered  flask,  add  50  mils  of  deci- 
normal  bromin  V.S.,  and  dilute  with  50  mils  of  distilled  water. 
Then  add  5  mils  of  hydrochloric  acid  and  at  once  stopper 
the  flask,  shake  well,  dilute  it  with  20  mils  of  distilled  water, 
add  5  mils  of.  potassium  iodid  T.S.,  let  stand  five  minutes,  and 
then  titrate  the  liberated  iodin  with  decinormal  sodium  thio- 
sulphate  V.S.,  using  starch  as  indicator. 

Calculate  as  in  preceding  assay.  Each  mil  of  decinormal 
bromin  corresponds  to  0.001834  gm.  of  C6H4(OH)2. 

Phenylsulphonates,  Sulphocarbolates.  Dissolve  about  0.25 
gm.  of  the  salt,  accurately  weighed,  in  50  mils  of  distilled 
water,  add  50  mils  of  decinormal  bromin  V.S.  and  5  mils  of 
hydrochloric  acid.  Allow  the  mixture  to  stand  for  fifteen 
minutes,  then  add  2  gms.  of  potassium  iodid  dissolved  in  5 
mils  of  water,  and  titrate  the  liberated  iodin  with  decinormal 
sodium  thiosulphate  V.S.,  using  starch  as  indicator. 

Each  mil  of  decinormal  bromin  V.S.  used  corresponds  to 
0.04903  gm.  of  NaCeHsO.SOs. 


CHAPTER  XIV 

SOME    TECHNICAL    EXAMINATION    METHODS    FOR    FATS, 
OILS  AND  WAXES 

The  Acid  Value  or  Proportion  of  Free  Fatty  Acids.  This 
indicates  the  number  of  milligrams  of  KOH  required  to  neutral- 
ize the  free  fatty  acids  in  i  gm.  of  oil,  fat  or  v/ax.    This  standard 

N  N 
alkali  used  is  in  alcoholic  solution  and  may  be  — ,  — ,  or  weaker, 

depending  upon  the  nature  of  the  fat.  Phenolphthalein  is  the 
indicator.  The  fat  is  dissolved,  according  to  Geissler,  in  ether, 
but  alcohol  or  purified  methylated  spirit,  chloroform  or  a  mix- 
ture of  alcohol  and  ether  may  be  used.  The  solverft,  wh'chever 
is  used,  must  be  free  from  acidity,  and  should  be  neutralized 

N 
with  —  alkali  if  necessary. 

The  Process.  Ten  gms.  of  the  oil  are  accurately  weighed 
into  a  flask  and  about  50  mils  of  solvent  added.  A  few  drops 
of  phenolphthalein   are   then   added   and   the   titration   with 

N  . 

alcoholic  —  potassium  hydroxid  solution  begun,  shaking  con- 
stantly until  the  first  appearance  of  a  pink  color.  Care  must 
be  taken  not  to  add  too  great  an  excess  of  the  alkali,  other- 
wise saponification  will  occur.  A  small  excess  may,  however, 
be  added  and  retitrated  with  standard  acid;  a  more  distinct 
end-point  is  then  obtained.  In  the  case  of  waxes  or  solid 
fats,  the  solvent  is  added,  heat  applied  until  it  boils,  and  the 
titration  at  once  started. 

277 


278     THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

N 
One  mil  of  —  KOH  =  0.02805  g"^-  ^^  KOH  or  28.05  mgms. 

The  number  of  mils  used,  multiplied  by  28.05  and  then 
divided  by  the  weight  of  oil  taken,  gives  the  milligrams  of 
KOH  neutralized  by  the  free  fatty  acids  of  the  oil,  i.e.,  the 
acid  value. 

The  Saponification  Value  (Kottstorfer  Number).*  This 
indicates  the  number  of  milligrams  of  KOH  required  for  the 
complete  saponification  of  one  gram  of  fat  or  oil.  Reagents 
required  are: 

Alcoholic  Potassium  HydrJxid  Solution,  made  by  dissolving 
40  gms.  of  potassium  hydroxld  in  i  liter  of  95  per  cent  alcohol. 

Half -Normal  Hydrochloric  Acid  Solution.  Each  mil  of 
which  =  28.05  rngms.  of  KOH. 

Indicator.  Phenol phthalein  i  gm.  in  ico  mils  of  95  per 
cent  alcohol. 

The  Prcfcess.  Into  an  Erlenmeyer  flask  capable  of  holding 
2CO  mils,  accurately  weigh  about  1.5  gms.  of  the  fat  (previously 
purified  and  filtered).  Run  into  this  from  a  burette  25  mils 
of  the  alcoholic  potassium  hydroxid  solution.  Then  insert 
into  the  neck  of  the  flask  a  perforated  stopper  provided  with 
a  glass  tube  70  to  80  cm.  in  length  and  from  5  to  8  mm.  in 
diameter,  and  set  it  in  a  water-bath  for  half  an  hour  or  until 
the  fat  is  entirely  saponified.  The  operation  is  facilitated 
by  occasional  agitation.  The  flask  is  then  removed,  its  con^ 
tents  cooled  and  titrated  with  the  half-normal  hydrochloric 
acid,  using  phenolphthalein  as  indicator.  The  alcoholic  potas^ 
slum  hydroxid  solution  is  standardized  by  conducting  a  blank 
experiment  similar  in  every  detail  to  the  above  with  the  exception 
that  the  fat  is  omitted. 

*  J.  Kottstorfer,  1879,  Zeitschr.  anal.  Chem.,  XVIII,  199. 


TECHNICAL  EXAMINATION  METHODS  FOR  FATS   279 

The  Kottstorfer  number  is  then  ascertained  by  subtracting 
the  number  of  mils  of  the  standard  hydrochloric  acid  used  in 
the  analysis  from  the  number  necessary  to  neutralize  25  mils 
of  the  alcoholic  potassium  hydroxid  solution  in  the  blank 
experiment,  multiplying  the  difference  by  28.05  and  dividing 
by  the  weight  of  fat  taken. 

Example.  1.5  gms.  of  the  fat  was  treated  with  25  mils  of 
alcoholic  potassium  hydroxid  solution,  under  the  above  described 

N 
conditions,  and  in  titrating  the  excess,  21.5  mils  of  —  hydro- 
chloric acid  was  required.     In  the  blank  experiment  25  mils 
of  the  alcoholic  potassium  hydroxid  required  32  mils  of  the 

N 

—  acid.     Then  32  — 21.5  =  10.5. 


10.5X28.05 
1^5 


=  196. 


196  is  the  milligrams  of  KOH  neutralized  by  i  gm.  of  the 
oil,  or  the  Kottstorfer  saponification  number. 

TABLE    SHOWING     REQUIREMENT    AS    TO     SAPONIFICATION 

NUMBER 


Lard  oil 195  to  197 

Almond  oil  (expressed)  .  .  191  to  200 

Cottonseed  oil 191  to  196 

Linseed  oil    18710195 

Cod-liver  oil 175  to  185 


Olive  oil 191  to  195 

Castor  oil 179  to  180 

Theobroma  oil 188  to  195 

Croton  oil 212  to  218 

Yellow  wax ^ 90  to    96 


The  Reichert  Number  or  Volatile  Fatty  Acid  Value.  This 
indicates  the  number  of  mils  of  decinormal  KOH  required  to 
neutralize  the  volatile  fatty  acids  distilled  from  2.5  gms.  of 
fat.  This  method  is  conducted  as  follows:  2.5  gms.  of  the 
clear  filtered  fat  are  taken  in  an  Erlenmeyer  flask  of  about 
150  mils  capacity  with  i  gm.  of  potassium  hydroxid  and  20  mils 


280      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

of  80  per  cent  alcohol,  and  the  whole  digested  on  a  water-bath 
(rotating  the  flask  frequently)  until  saponification  is  complete 
and  the  alcohol  all  removed.  50  mils  of  water  are  then  added, 
and  20  mils  of  dilute  sulphuric  acid  (1:10)  and  the  mixture 
distilled.  The  distillate  is  collected  in  a  50-mil  flask  into 
which  is  set  a  funnel  with  a  wetted  filter  to  collect  any  insoluble 
fat  acid.  The  first  10  or  20  mils  of  distillate  are  returned  to 
the  flask,  and  the  50  mils  distilled.  This  is  treated  with  a  few 
drops  of  phenolphthalein  and  then  titrated  with  decinormal 
potassium  hydroxid.  The  number  of  mils  consumed  is  the 
Reichert  Number. 

In  the  Meissl  modification  (the  Reicliert-Meissl  method) 
5  gms.  of  the  fat  are  taken  and  a  more  complete  distillation 
of  the  volatile  acids  effected.  The  method  is  particularly 
useful  in  determining  the  genuineness  and  purity  of  butter. 

The  Reichert-Meissl  Number.  This  is  undoubtedly  the 
best  process  for  detecting  the  admixture  of  foreign  fats  with 
butter.  This  process  depends  upon  the  fact  that  butter  con- 
tains certain  constituents  which,  when  appropriately  treated, 
yield  volatile  acids  in  much  larger  quantity  than  is  obtained 
from  any  of  the  practicable  substitutes  for  butter.  These 
acids  are  principally  butyric  and  caproic.  The  process  con- 
sists in  saponifying  the  fat  with  an  alkali,  then  separating 
the  fatty  acids  by  neutralization,  and  distilling  off  the  volatile 
acids  for  titration  with  standard  alkali. 

The  operations  involved  in  this  process  do  not  admit  of 
any  arbitrary  variation,  and  reliable  and  comparable  results 
can  only  be  obtained  by  strictly  adhering  to  the  prescribed 
details. 

The  following  process  is  adopted  by  the  Association  of 
Official  Agricultural  Chemists.     The  solutions  required  are: 

Sodium  Hydroxid  Solution.  One  hundred  gms.  of  sodium 
hydroxid  are  dissolved  in  100  mils  of  distilled   water.    The 


TECHNICAL  EXAMINATION  METHODS  FOR  FATS   281 

alkali  should  be  as  free  as  possible  from  the  carbonates,  and 
be  preserved  out  of  contact  with  the  air. 

Potassium  Hydroxid  Solution.  One  hundred  gms.  of  the 
purest  potassium  hydroxid  are  dissolved  in  58  mils  of  hot  dis- 
tilled water,  cooled  in  a  stoppered  vessel,  and  the  clear  liquid 
decanted  and  preserved  out  of  contact  with  the  air. 

Sulphuric  Acid.  Mix  2co  mils  of  the  strongest  acid  with 
1000  mils  of  water. 

Alcohol  of  about  95  per  cent,  redistilled  from  caustic  soda. 

Standard  Barium  Hydroxid  Solution,  accurately  standard- 
ized, approximately  decinormal. 

Indicator.  Dissolve  i  gm.  of  phenolphthalein  in  100  mils 
of  95  per  cent  alcohol. 

The  process: 

Weighing  the  Butter.  The  butter  to  be  examined  should 
be  melted  and  kept  in  a  dry,  warm  place  at  about  60°  C.  for 
two  or  three  hours,  until  the  water  and  curd  have  entirely 
settled  out.  The  clear  supernatant  fat  is  poured  off  and 
filtered  through  a  dry  filter-paper  in  a  jacketed  funnel  con- 
taining boiling  water.  Should  the  filtered  fat  in  a  fused  state 
not  be  perfectly  clear,  it  must  be  filtered  a  second  time.  This 
is  to  remove  all  foreign  matter  and  any  trace  of  moisture. 
The  saponification  flasks  are  prepared  by  thoroughly  washing 
with  water,  alcohol  and  ether,  wiping  perfectly  dry  on  the 
outside,  and  heating  for  one  hour  at  the  temperature  of  boiling 
water.  The  flasks  should  then  be  placed  in  a  tray  by  the 
side  of  the  balance  and  covered  with  a  silk  handkerchief  until 
they  are  perfectly  cool.  They  must  not  be  wiped  with  a  silk 
handkerchief  within  fifteen  or  twenty  minutes  of  the  time 
they  are  weighed.  The  weight  of  the  flasks  having  been 
accurately  determined,  they  are  charged  with  the  melted  fat 
in  the  following  way: 

A  pipette  with  a  long  stem,  marked  to  deliver  5.75  mils, 


282      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 


is  warmed  to  a  temperature  of  about  50°  C.  The  fat,  having 
been  poured  back  and  forth  once  or  twice  into  a  dry  beaker 
in  order  to  thoroughly  mix  it,  is  then  taken  up  in  the  pipette 
and  the  nozzle  of  the  pipette  carried  nearly  to  the  bottom  of  the 
flask,  having  been  previously  wiped  to  remove  any  adhering 
fat,  and  5.75  mils  of  fat  are  allowed  to  flow  into  the  flask. 
After  the  flasks  have  been  charged  in  this  way  they  should 
be  recovered  with  the  silk  handkerchief,  allowed  to  stand 
fifteen  or  twenty  minutes,  and  again  weighed. 

The  Saponification.    Three  methods  may  be  employed: 

1.  Under  Pressure  with  Alcohol.     Ten  mils  of  95  per  cent 

alcohol  are  added  to  the  fat  in 
the  flask,  and  then  2  mils  of  the 
caustic  soda  solution.  A  soft 
cork  stopper  is  now  inserted  in 
the  flask  and  tied  down  with 
a  piece  of  twine.  The  saponi- 
fication is  then  completed  by 
placing  the  flask  upon  the 
water  or  steam  bath  (see  Fig. 
55).  The  flask  during  the 
saponification,  which  should 
last  one  hour,  should  be 
gently  rotated  from  time  to 
time,  being  careful  not  to  pro- 
ject the  soap  for  ^ny  distance 
up  its  sides.  At  the  end  of 
an  hour  the  flask,  after  having 
been  cooled  nearly  to  room 
temperature,  is  opened. 

2.  Under  Pressure  without  the  Use  of  Alcohol.  Place  2 
mils  of  the  potassium  hydroxid  in  the  flask  containing  the  fat, 
which  must  be  round-bottomed  and  made  of  we  11- annealed 


Fig.  55. 


TECHNICAL  EXAMINATION  METHODS  FOR  FATS    283 

glass  to  resist  the  pressure;  cork,  and  heat  as  in  the  pievious 
method.  Rotate  the  flask  very  gently  during  the  saponifica- 
tion, taking  great  care  that  none  of  the  fat  rises  on  the  sides 
of  the  flask  out  of  reach  of  the  alkaH.  Potash  makes  a  softer 
soap  than  soda  and  thus  allows  a  complete  saponification 
without  the  use  of  alcohol.  This  method  avoids  the  danger 
of  formation  of  esters  and  the  trouble  of  removing  the  alcohol 
after  saponification. 

3.  With  a  Reflux  Condenser  and  the  Use  of  Alcohol.  Place 
10  mils  of  the  95  per  cent  alcohol  in  the  flask  containing  the 
fat,  add  2  mils  of  the  sodium  hydroxid  solution  with  a  reflux 
condenser  (a  glass  tube  not  less  than  i  mxCter  in  length  is 
allowable)  and  heat  on  the  steam  bath  until  the  saponification 
is  complete. 

Removal  of  the  Alcohol.  The  stoppers  having  been  laid 
loosely  in  the  mouth  of  the  flask,  the  alcohol  is  removed  by 
dipping  the  flask  into  a  steam  bath.  The  steam  should  cover 
the  whole  of  the  flask  except  the  neck.  After  the  alcohol  is 
nearly  removed,  frothing  may  be  noticed  in  the  soap,  and 
to  avoid  any  loss  from  this  cause  or  any  creeping  of  the  soap 
up  the  sides  of  the  flask,  it  should  be  removed  from  the  bath 
and  shaken  to  and  fro  until  the  frothing  disappears.  The 
last  traces  of  alcohol  vapor  may  be  removed  from  the  flask 
by  waving  it  briskly,  mouth  down,  to  and  fro,  or  better,  by 
a  current  of  carbon  dioxid  free  air. 

Dissolving  the  Soap.  After  the  removal  of  the  alcohol 
the  soap  should  be  dissolved  by  adding  135  mils  of  recently 
boiled  distflled  water  (or  132  mils  if  potassium  hydroxid  was 
used  in  the  saponification),  warming  on  the  steam  bath,  with 
occasional  shaking  until  solution  of  the  soap  is  complete. 

Setting  Free  the  Fatty  Acids  and  Distilling.  Cool  to  from 
60°  to  70°  C,  throw  in  a  few  pieces  of  pumice  stone,  add  5 
mils  of  the  dilute  sulphuric  acid  (or  8  mils  if  potassium  hydroxid 


284      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

was  used  in  the  saponification),  stopper  as  in  the  method  of 
saponification,  and  heat  on  the  water-balh  until  the  fatty 
acids  form  a  clear,  transparent  layer  on  top  of  the  water. 
This  may  take  several  hours.  Cool  to  room  temperature, 
add  a  few  pieces  of  pumice  stone,  and  connect  with  a  glass 
condenser,  as  in  Fig.  56. 

Heat  slowly  with  a  naked  flame  until  ebullition  begins, 
and  distil,  regulating  the  flame  in  such  a  way  as  to  collect  no 
mils  of  distillate  in  as  nearly  thirty  minutes  as  possible. 

Mix  this  distillate,  filter  through  a  dry  filter,  and  titrate 
100  mils  with  the  standard  barium  hydroxid  solution,  using 


Fig.  56. 

0.5  mil  of  phenolphthalein  as  indicator.  The  red  color  should 
remain  unchanged  for  two  or  three  minutes. 

Increase  the  number  of  cubic  centimeters  of  tenth-normal 
alkali  used  by  one- tenth,  divide  by  the  weight  of  fat  taken, 
and  multiply  by  five  to  obtain  the  Reichert-Meissl  number. 
Correct  the  result  by  the  figure  obtained  in  a  blank  exper- 
iment. 

WTien  treated  as  above  described,  5  gms.  of  genuine  butter 

N 
never  yields  less  acidity  than  is  represented  by  24  mils  of  — 


TECHNICAL  EXAMINATION  METHODS  FOR  FATS    285 

alkali.  It  is  true,  however,  that  the  butter  made  from  the 
milk  of  a  single  cow,  especially  towards  the  end  of  her  period 
of  lactation,  has  been  known  to  fall  slightly  below  this  figure, 
but  the  average  butter,  as  produced  from  the  mixed  milk  of 
a  herd,  usually  requires  27  mils  or  more.  Oleomargarin  re- 
quires about  I  mil  of  beef -fat,  and  lard  about  the  same.  Cacao 
butter  requires  about  7  mils. 

The  percentage  of  butter-fat  in  a  mixture  of  fats, 
5  gms.  being  taken:  (w  -  0.6)  X  3.65  =  percentage  of  true 
butter-fat. 

lodin  Absorption  Number  of  Fats  and  Oils  (Hiibl's  Num- 
ber.)* This  is  the  percentage  of  iodin  absorbed  by  a  fat 
or  an  oil  under  certain  conditions.  In  other  words  it  is  the 
number  of  parts  of  iodin  absorbed  by  100  parts  of  an  oil. 

Reagents  required : 

{a)  Hiibl's  Iodin  Solution.  Dissolve  25  gms.  of  pure  iodin 
in  500  mils  of  alcohol,  and  mix  this  solution  with  500  mils  of 
alcohol  containing  30  gms.  of  pure  mercuric  chlorid,  and  set 
aside  for  twenty-four  hours.  The  mercuric  chlorid  solution 
should  be  filtered  if  necessary  before  it  is  mixed  with  the 
alcoholic  iodin  solution.  This  solution  loses  its  strength 
rapidly,  and  should  therefore  be  tested  before  using. 

Q))  Decinormal  sodium  thiosulphate. 

(c)  Potassium  iodid  solution,  20  gms.  in  100  mils. 

{d)  Starch  paste  indicator. 

The  Process.  To  about  0.3  gm.t  of  the  fat  or  oil,  accu- 
rately weighed  and  dissolved  in  10  mils  of  chloroform,  con- 

*  Dingler's  Polyt.  Jour.,  1884,  281;  Am.  Ch.  Jour.,  VI,  285. 

t  In  the  case  of  drying  oils  which  have  a  very  high  absorbent  power,  as 
linseed  oil,  use  from  0.15  to  0.20  gm.;  for  oil  of  theobroma  and  similar  fats 
use  0.80  gm. 


286      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

tained  in  a  glass-stoppered  bottle  of  250  mils  capacity,  add  25 
mils  of  the  iodin  solution.  Stopper  the  bottle  securely  and 
place  in  a  cool,  dark  place  for  four  hours.*  At  the  expiration 
of  this  time,  the  mixture  must  still  possess  a  brown  color;  if 
it  does  not,  a  further  measured  quantity  of  the  iodin  solution 
must  be  added  and  the  mixture  again  set  aside.  Finally  20 
mils  of  the  potassium  iodid  solution  are  added  together  with 
50  mils  of  water,  and  the  mixture  titrated  with  the  decinormal 
sodium  thiosulphate  until  the  color  is  almost  discharged, 
when  a  few  drops  of  starch  indicator  are  added  and -the 
titration  continued  until  the  solution  is  colorless. 

The  Standardization  of  the  Iodin  Solution  is  effected  by 
subjecting  it  to  the  same  treatment  as  in  the  assay,  but  with 
the  oil  omitted,  and  at  the  end  of  four  hours  titrating  with 
tlie  decinormal  thiosulphate. 

The  difference  between  the  number  of  mils  of  thiosulphate 
solution  used  in  the  blank  test  and  the  number  used  in  the 
actual  assay,  is  multiplied  by  12.59,  ^^^  ^^i^  divided  by  3, 
gives  the  iodin  value  of  the  oil  under  analysis. 

When  the  quantity  of  the  oil  or  fat  taken  is  not  0.3  gm., 
then  the  product  is  not  divided  by  3,  but  by  the  figure  corre- 
sponding to  the  quantity  taken;  thus  if  0.15  gm.  are  taken 
the  product  is  divided  by  1.5, 

Another  way  of  making  the  calculation  is  as  follows: 

The  difference  between  the  mils  of  thiosulphate  used  in 
the  blank  test  and  the  mils  used  in  the  analysis,  is  multiplied 
by  0.01259,  then  by  100,  and  the  product  divided  by  the 
weight  in  grams  of  the  oil  taken  for  analysis. 

*  The  time  allowed  does  not  give  the  complete  iodin  absorption  power 
of  an  oil  or  fat  and  cannot  be  compared  with  determinations  in  which  six  to 
twelve  hours  have  been  used.  It  gives,  however,  very  satisfactory  comparative 
results,  but  the  time  factor  must  be  very  closely  observed. 


TECHNICAL  EXAMINATION  METHODS  FOR  FATS     287 

Example: 

Number  of  mils  of  thiosulphate  used  in  blank  test 45 .4 

"     "     "  "  "      "analysis 26.1 


representing  iodin  absorbed 19.3 


.    19.3  X0.01259X  100 

lodm  number  is =  80.9. 

0.3 

This  method,  as  is  well  known,  is  based  upon  the  fact 

that  the  unsaturated  glycerids  in  the  oils  form  addition  products 

with  the  iodin.     The  mercuric  chlorid  and  iodin  contained 

in  the  alcoholic  solution  interact  with  a  formation  of  mercuric 

chloriodid  and  iodin  chlorid;    the  latter  is  supposed  to  be 

the  active  agent. 

HgCl2  +  l2=HgClI+ICl. 

The  mercuric  chlorid  also  acts  the  part  of  a  carrier  of  halogen 
similar  to  that  played  by  mercury  in  the  Kjeldahl  process 
when  dissolving  the  substance  in  sulphuric  acid. 

Gill  and  Adams,  J.  A.  C.  S.,  xxii,  12,  call  attention  to 
the  fact  that  not  only  addition  products,  but  also  substitution 
products,  are  formed  in  this  process,  which  vary  in  amount 
with  the  time  of  action  and  the  strength  of  the  solution.  This 
is  a  feature  which  interferes  with  the  accurate  determination 
of  the  iodin  number,  giving  as  it  does  a  higher  figure.  In 
order  to  prevent  this  formation  of  substitution  products,  and 
thus  overcome  the  discrepancy  in  results,  these  authors  suggest 
the  use  of  mercuric  iodid  instead  of  mercuric  chlorid,  making 
the  solution  with  methyl  alcohol  (free  from  acetone  and 
anhydrous).  They  claim  that  by  the  use  of  a  solution  so 
made  a  truer  iodin  absorption  number  is  obtained.  A  great 
disadvantage  of  the  Hiibl  method  is  that  the  solution  quickly 
loses  its  strength;    another  is,  the  length  of  time  required 


288      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

for  the  absorption.  As  above  described,  four  hours  are  required, 
but  this  is  not  sufficient  time  to  allow  of  complete  absorption 
of  the  iodin.  It  is,  however,  a  good  practice  to  have  a  fixed 
time  limit;  the  process  then  gives  very  satisfactory  comparative 
results. 

Iodin  Absorption  Number.  The  Hanus  Method.*  This 
method  is  conducted  like  the  foregoing.  It  differs  in  the  com- 
position of  the  iodin  solution  used.  This  is  prepared  as  fol- 
lows: Dissolve  13.2  gms.  of  powdered  iodin  in  1000  mils  of 
pure  glacial  acetic  acid  with  the  aid  of  gentle  heat  if  necessar}', 
to  facilitate  solution.  Cool  to  25°  C.  and  determine  the  iodin 
content  in  20  mils  of  the  solution  by  means  of  decinormal 
sodium  thiosulphate  V.S.;  then  add  to  the  solution  a  quantity 
of  bromin,  molecularly  equivalent  to  that  of  the  iodin  present 
(3  mils  is  the  usual  approximate  amount).  Keep  the  solution 
in  glass-stoppered  bottles  protected  from  .light. 

The  Process.  Introduce  about  0.8  gm.  of  a  solid  fat  or 
0.3  gm.  of  an  oil,  accurately  weighed,  into  a  glass-stoppered 
bottle  of  250  mils  capacity.  Dissolve  it  in  10  mils  of  chloro- 
form, add  25  mils  of  the  iodin  solution,  stopper  the  bottle 
securely,  and  allow  the'  mixture  to  stand  for  half  an  hour 
in  a  cool,  dark  place.  After  this  time  it  must  still  retain  a 
brown  color;  if  it  is  not  brown,  a  new  test  should  be  started 
using  a  smaller  quantity  of  the  fat  or  oil.  Then  add  in  the 
order  named,  30  mils  of  potassium  iodid  T.S.,  100  mils  of 
distilled  water,  and  decinoimal  sodium  thiosulphate  V.S.  in 
small  successive  portions,  shaking  thoroughly  after  each  addi- 
tion, until  the  color  of  the  mixture  becomes  quite  pale.  Then 
add  a  few  drops  of  starch  solution  and  continue  the  addition 
of  the  thiosulphate  V.S.  until  the  blue  color  is  discharged. 
Then  make  a  blank  test  by  mixing  exactly  the  same  quantities 

*  Zeitschr.  Nahr.  u.  Genus.  (1901),  913. 


TECHNICAL  EXAMINATION  METHODS   FOR  FATS     289 

of  the  iodin  solution  and  chloroform,  and  titrating  the  free 
iodin  with  thiosulphate  V.S.  as  directed  above.  The  dif- 
ference in  the  number  of  mils  of  the  thiosulphate  V.S.  con- 
sumed in  the  blank  test  and  the  actual  test  multiplied  by  1.269 
and  divided  by  the  weight  of  the  fat  or  oil  taken  gives  the  iodin 
number. 

In  the  case  of  oils  which  have  a  high  iodin-absorbing 
power,  smaller  quantities  of  the  oils  should  be  used.  0.15  to 
0.18  gm.  for  linseed  oil  and  about  0.2  gm.  for  cod  liver  oil. 
The  lime  allowed  for  absorption  in  the  case  of  castor  oil  and 
linseed  oil  should  be  one  hour. 

The  consensus  of  opinion  among  chemists  is  that  the  Hanus 
method  is  the  most  satisfactory.  Its  principal  advantage  over 
the  Hiibl  method  lies  in  the  facts  (a)  that  the  Hanus  iodin 
solution  is  much  more  stable,  (b)  that  the  time  requu-ed  for 
the  reaction  is  comparatively  much  shorter. 


TABLE    SHOWING    IODIN    ABSORPTION    NUMBER    FOR    SOME 
COMMON  OILS 


Hiibl's  No. 
Four  Hours. 


Almond  oil  (expressed) 98 

Butter 35 

Castor  oil 87 

Cocoanut  oil 8 

Cod  liver  oil 144 

Cottonseed  oil 104 

Lard  oil 69 

Linseed  oil 1 79 

Mustard  oil 113 

Olive  oil 86 

Oleomargarin 52 

Peanut  oil ' 96 

Rape  oil 102 

Sesiime  oil 106 

Theobroma  oil S5 


Hanus'  No. 
Half  an  Hour. 

7 
4 
5 
6 

9 
6 

7 
2 

3 
8 

3 
3 
2 

5 

5 


.  2 

98 

•4 

35 

•3 

87 

•4 

8 

•3 

143 

■3 

106 

■5 

69 

.6 

186 

.  I 

115- 

.1 

86. 

.  I 

52. 

•3 

97- 

•4 

105. 

4 

106. 

•4 

35- 

290     THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

The  Acid  Number  for  Resins.  This  is  the  number  of 
milligrams  of  potassium  hydroxid  required  to  neutralize  i 
gm.  of  a  resinous  substance. 

The  Process.  About  2  gms.  of  the  resinous  substance, 
accurately  weighed,  is  dissolved  in  alcohol,  and  the  solution 
titrated  with  half-normal  potassium  hydroxid  V.S.,  using  phenol- 
phthalein  as  indicator.  The  number  of  mils  of  the  half- 
normal  alkali  V.S.  required  for  neutralization  when  multiplied 
by  0.028055  and  the  product  divided  by  the  weight,  gives  the 
acid  number  of  the  resin. 


CHAPTER  XV' 
ESTIMATION  OF  SUGARS 

Preparation  of  Fehling's  Solution,  {a)  The  Copper  Solu- 
tion. 34.66  gms.  of  carefully  selected  small  crystals  of  pure 
cupric  sulphate  are  dissolved  in  sufficient  water  to  make,  at 
or  near  25°  C,  exactly  500  mils.  Keep  in  small  well-stoppered 
bottles. 

(b)  The  Alkaline  Tartrate  Solution.  173  gms.  of  potassium 
and  sodium  tartrate  (Rochelle  salt)  and  75  gms.  of  potassium 
hydroxid  are  dissolved  in  sufficient  water  to  make,  at  or  near 
25°  C,  exactly  500  mils.  Keep  in  small  rubber-stoppered 
bottles. 

For  use,  equal  quantities  of  the  two  solutions  should  be 
mixed  at  the  time  required. 

One  molecular  weight  of  water-free  glucose  will  reduce 
five  molecular  weights  of  cupric  oxid,  i.e.,  180.12  gms.  of 
glucose  will  reduce  1248.7  gms.  of  crystallized  copper  sul- 
phate (CUSO4  +  5H2O).  If  pure  chemicals  are  used  in  the 
preparation  of  this  solution  there  will  be  no  need  of  standard- 
izing it,  but  if  the  solution  is  old  and  its  titer  doubtful,  the 
following  method  of  standardization  may  be  employed. 

The  Standardization.  Dissolve  0.95  gm.  of  pure  cane 
sugar  in  50  mils  of  water,  add  2  mils  of  hydrochloric  acid,  and 
heat  to  70°  C.  for  ten  minutes.  Then  neutralize  with  sodium 
carbonate  and  dilute  to  i  liter. 

Fifty  mils  of  this  solution  should  exactly  reduce  10  mils 
of  Fehling's  solution,  when  treated  as  described  below. 

291 


292      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

Ten  mils  of  the  mixed  Fehling's  solution  is  equivalent  to 

Glucose 0.050    gm. 

Maltose 0.0806    " 

Inverted  cane-sugar 0.0475    " 

Inverted  starch 0.045      '' 

Lactose 0.0678   " 

Estimation  of  Reducing  Sugars.  {Glucose,  Maltose,  In- 
verted Cane-sugar,  Lactose  and  Inverted  Starch.  0.5  gm.  or 
less  of  the  sugar  is  dissolved  in  100  mils  of  distilled  water, 
and  put  into  a  burette.  Ten  mils  of  the  mixed  Fehling's  solu- 
tion, accurately  measured,  is  put  into  a  loo-mil  Erlenmeyer 
flask,  or  in  a  white  porcelain  dish,  diluted  with  40  mils  of 
distilled  water,  and  rapidly  heated  to  boiling.  The  solution 
should  remain  perfectly  clear  and  retain  its  blue  color.  If 
it  does  not  it  should  be  rejected. 

The  hot,  clear  blue  solution  is  immediately  titrated  with 
the  sugar  solution,  which  should  be  added  in  small  portions  at 
a  time,  boiling  after  each  addition  until  the  copper  is  com- 
pletely precipitated  and  the  blue  color  of  the  solution  is  entirely 
destroyed.  The  solution  is  then  boiled  about  two  minutes 
longer,  and  the  amount  of  the  sugar  solution  used  is  read  off. 

The  Calculation.  Ten  mJls  of  Fehling's  solution  are  always 
taken;  and  whatever  the  quantity  of  glucose  or  other  sugar 
solution  is  required  to  effect  its  complete  reduction,  that 
quantity  contains  the  equivalent  of  10  mils  of  Fehling's  solu- 
tion, as  shown  in  the  table  above. 

Thus  if  12  mils  of  the  sugar  solution  were  required  to  reduce 
10  mils  of  Fehling's  solution,  the  12  mils  contain  0.05  gm.  of 
glucose  or  0.0806  gm.  of  maltose,  etc.  One  hundred  mils  of 
the  solution  therefore  contain  x  gm.  of  glucose. 


0.05  X 100  ^  , 

— _^_ =  0.416  "^m.  glucose. 

12  T     .        O  O 


ESTIMATION  OF  SUGARS  293 

In  order  to  obtain  reliable  results  it  is  important  that  the 
process  be  carried  out  exactly  as  laid  down  in  the  above 
directions,  and  that  the  quantity  of  sugar  present  in  solution 
be  no  greater  than  one  per  cent.  The  degree  of  heat  and 
the  time  occupied  in  the  process,  as  well  as  the  concentration 
of  the  Fehling's  solution,  have  a  very  important  bearing  upon 
the  accuracy  of  the  results. 

It  is  advisable  to  complete  the  titration  in  as  short  a  time 
as  possible.  A  preliminary  test  should  always  be  made,  in 
which  the  approximate  quantity  of  the  solution  required  is 
found;  then  a  second  and  more  accurate  titration  can  be 
done  in  which  the  sugar  solution  may  be  added  more  boldly, 
and  the  time  of  boiling  and  exposure  of  the  copper  solution 
to  the  air  much  lessened.  The  complete  reduction  of  the 
copper  (using  undiluted  Fehling's),  after  the  addition  of  the 
requisite  quantity  of  sugar,  does  not  take  place  instantly. 
The  time  required  varies  somewhat  with  the  different  sugars. 
For  instance,  with  glucose,  invert  sugar  and  levulose,  the 
reduction  is  not  complete  until  after  heating  two  minutes; 
with  maltose  four  minutes,  and  with  lactose  six  minutes  are 
required. 

//  the  sugar  to  he  examined  he  either  glucose,  maltose,  or 
lactose,  it  may  be  titrated  du-ectly;  but  if  it  ht^ sugar-cane,  it 
must  first  be  inverted.  This  is  done  by  dissolving  the  sugar 
(0.5  gm.)  in  about  100  mils  of  water,  adding  3  or  4  drops  of 
strong  hydrochloric  acid,  and  boiling  briskly  for  ten  or  fifteen 
minutes.  This  is  then  allowed  to  cool,  neutralized  with  potas- 
sium hydroxid,  and  made  up  to  100  mils  with  water. 

The  sugar  in  urine  may  he  estimated  hy  this  process.  The 
urine  is  placed  in  the  burette  and  run  into  the  boiling  Fehling's 
solution  in  the  usual  manner.  If  it  contain  a  large  quantity 
of  sugar,  it  must  be  diluted  two  or  three  times. 

In  estimating  with  Fehling's  solution  it  is  well  to  attach  a 


294       THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

rubber  tube  eight  to  twelve  inches  in  length  to  the  lower  end 
of  the  burette,  so  that  the  boiling  need  not  be  done  directly 
under  the  burette,  and  thus  cause  incorrect  readings  through 
the  expansion  of  the  liquid  therein. 

Determination  of  the  End-point.  It  is  always  somewhat 
difficult  to  determine  the  exact  point  at  which  the  blue  color 
disappears,  owing  to  the  presence  of  the  precipitated  suboxid 
of  copper.  This  difficulty  may  be  overcome  by  the  addition 
of  some  substance  which  will  prevent  the  precipitation  of 
the  cuprous  oxid,  such  as  ammonium  hydroxid  or  potassium 
ferrocyanid.  When  the  latter  is  used  the  disappearance  of 
the  blue  color  can  then  be  readily  seen,  as  the  solution  remains 
clear  to  the  end,  turning  from  blue  to  green,  and  finally  brown, 
which  indicates  the  end  of  the  reaction. 

Professor  Bartley  reports  this  method  as  accurate,  reliable, 
and  rapid,  provided  the  solution  be  not  boiled  during  the 
reduction.  He  recommends  adding  to  the  Fehling's  solution 
in  the  porcelain  basin  lo  mils  of  a  lo  per  cent  freshly  prepared 
solution  of  potassium  ferrocyanid  and  30  mils  of  water.  The 
ferrocyanid  does  not  precipitate  the  copper  in  alkaline  solution. 

L.  Beulaygue  (Compt.  rend.,  138,  51)  suggests  the  following 
method  for  determining  the  end-reaction  when  titrating  sugar 
solution  in  the  usual  manner  with  Fehling's  reagent,  using 
solution  of  sodium  monosulphid  as  the  indicator.  When  the 
end  of  the  reaction  is  near,  a  little  of  the  hot  solution  is  applied 
by  means  of  a  glass  rod,  to  two  superimposed  white  filter 
papers.  The  upper  one  acts  as  a  filter,  retaining  the  par- 
ticles of  cuprous  oxid.  The  lower  paper  is  withdrawn,  and  the 
moist  spot  touched  with  a  drop  of  the  sodium  monosulphid 
reagent,  when  an  immediate  black  stain  of  cupric  sulphid 
is  formed  if  the  reaction  is  not  complete.  By  successive 
spotting  out  and  testing  in  this  manner,  a  very  accurate  reading 
of  the  end-reaction  may  be  obtained.     It  is  important,  when 


ESTIMATION  OF  SUGARS  295 

standardizing  the  Fehling's  solution,  that  the  same  indicator 
should  be  employed.  Potassium  ferrocyanid  in  solution, 
acidified  by  either  hydrochloric  or  acetic  acid,  may  be  employed 
in  a  similar  manner.  The  end-reaction  in  then  indicated  by 
the  disappearance  of  the  red  color  from  the  last  spot. 

E.  F.  Harrison,*  who  has  employed  Fehling's  solution 
somewhat  extensively  in  quantitative  sugar  estimations,  has 
found  the  indicators  usually  recommended  to  determine  the 
end-point  of  reaction  to  be  unsatisfactory.  It  was  suggested 
by  him  that  the  action  of  cupric  salts  in  liberating  iodin  from 
iodid  might  be  utilized  for  this  purpose  with  advantage,  and 
his  experiments  determine  the  superiority  of  this  over  the 
other  indicators  heretofore  proposed.  The  indicator  is  pre- 
pared by  boiling  0.05  gm.  of  starch  with  a  few  mils  of  water, 
adding  10  gms.  of  potassium  iodid  and  diluting  to  100  mils. 
This  indicator  should  be  prepared  as  required.  In  use  0.5 
to  i.o  mil  of  this  solution  is  acidified  with  about  5  or  10 
drops  of  acetic  acid,  and  one  drop  or  more  of  the  liquid  in 
process  of  titration  added.  As  long  as  unreduced  copper  is 
present,  a  color  is  produced,  varying  from  red  to  blue,  and 
of  greater  or  less  intensity,  according  to  the  nearness  of  the 
end-point.  The  production  of  no  color  marks  the  end  of 
the  reaction.  The  indicator  is  available  with  one  drop  of  a 
solution  containing  one  part  of  cupric  sulphate  in  twenty 
thousand  parts. 

Estimation  of  Maltose  in  Malt  Extract.  A  half  per  cent 
solution  of  the  sample  is  prepared,  and  titrated  into  10  mils 
of  Fehling's  solution  in  the  manner  described  above. 

Estimation  of  Starch  After  Inversion.  This  method  con- 
sists in  converting  the  starch  into  glucose  and  then  estimating 
the  glucose  with  Fehling's  solution. 

Two  gms.  of  the  starch  are  mixed  with  water,  and  boiled 

*  Trans.  Brit.  Ph.  Conf.,  1903,  568-9. 


296     THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

for  fifteen  minutes;  the  solution  is  then  cooled,  and  diluted  to 
about  200  mils.  Eighteen  mils  of  strong  hydrochloric  acid 
are  then  added,  and  the  solution  heated  under  a  return  con- 
denser for  two  or  three  hours.  The  solution  is  then  cooled, 
neutralized  with  potassium  hydroxid  and  diluted  to  250  mils. 
This  is  put  into  a  burette  and  titrated  into  10  mils  of  Fehling's 
solution,  as  described  above,  under  reducing  sugars. 

In  estimating  the  starch  in  baking  powder,  2  to  5  gms. 
of  the  powder  are  introduced  into  an  Erlenmeyer  flask,  150 
to  200  mils  of  a  4  per  cent  solution  of  hydrochloric  acid  are 
added  and  the  solution  gently  boiled  for  four  hours,  after 
which  the  flask  and  contents  are  cooled,  neutralized  by  adding 
sodium  hydroxid,  and  made  up  to  a  definite  volume.  It  is 
then  ready  for  testing  with  Fehling's  solution. 

Ten  mils  of  Fehling's  solution  is  the  equivalent  of  0.047.-; 
gm.  of  invert  starch. 

Estimation  of  Starch  after  Inversion  by  Means  in  Diastase. 
The  treatment  of  starch  with  malt  infusion  or  pure  diastase 
at  a  temperature  not  above  70°  C,  readily  converts  the  starch 
into  maltose,  but  the  solution  also  contains,  besides  maltose, 
various  dextrins  in  proportions  varying  with  the  temperature 
at  which  the  diastase  acts.  The  digestion  may  vary  from 
fifteen  minutes  to  fifteen  hours.  Complete  conversion  of  the 
starch  may  be  determined  by  testing  occasionally  with  iodin. 
A  blank  experiment  should  be  made,  especially  if  the  digestion 
is  carried  on  beyond  half  an  hour.  A  like  quantity  of  the 
same  diastase  solution  should  be  digested  at  the  same  tem- 
perature and  for  the  same  time,  and  the  amount  of  sugar 
found  deducted  from  the  total  quantity  found  in  the  analysis. 
Faulenbach  *  makes  use  of  the  following  solution  of  diastase : 
Crush  3.5  kilos  of  fresh  green  malt,  treat  with  a  mixture  of 
two  Hters  of  water  and  four  liters  of  glycerin  and  let  stand 

*  Zeitschr,  f.  physiol.  Chem,,  VII,  510;  and  Chem.  Centralh,,  1883,  632. 


CALIFORNIA   COLLEfiE 

of   PHARMACY 


ESTIMATION  OF  SUGARS  297 

for  one  week,  stirring  occasionally;  then  express  and  filter. 
This  solution  is  very  stable.     Five  drops  of  it  will  dissolve 

1  gm.  of  starch;  15  drops  of  it  contain  a  quantity  of  carbo- 
hydrate =0.001  gm.  of  glucose. 

A  quantity  of  the  substance  to  be  tested  (containing  about 

2  gms.  of  starch)  is  boiled  to  gelatinize  the  starch.  Fifteen 
drops  of  the  diastase  solution  are  then  added,  and  the  mix- 
ture digested  at  63°  C.  It  is  then  filtered  to  separate  the 
undissolved  cellulose,  etc.,  and  heated  with  20  mils  of  hydro- 
chloric acid  in  a  water-bath  for  three  hours.  The  acidity 
is  then  just  destroyed  by  means  of  caustic  soda,  the  glucose 
determined,  o.ooi  gm.  deducted,  and  the  starch  calculated 
from  the  glucose. 

O ' Sullivan  *  employs  pure  diastase,  prepared  as  follows: 
Pour  sufficient  water  over  2  or  3  kilos  of  finely  crushed 
])ale  malt  to  just  cover  it.  Let  stand  for  three  or  four  hours, 
then  express  and  filter  the  solution.  Add  alcohol  (sp.gr.  0.83) 
until  the  liquid  above  the  flocculent  precipitate  becomes  opal- 
escent. Collect  the  precipitate,  wash  it  with  alcohol  (sp.gr. 
0.86  to  0.88)  then  with  absolute  alcohol  and  press  it  between 
linen.     Finally  dry  it  in  a  vacuum  over  sulphuric  acid. 

Estimation  of  the  Diastasic  Value  of  Malt  Extract.  The 
diastasic  value  of  malt  extracts  may  also  be  determined  by 
estimating  the  amount  of  maltose  produced  by  a  given  amount 
of  the  extract  in  a  given  time,  when  brought  in  contact  with 
an  excess  of  gelatinized  starch  solution.  It  is  always  necessary 
to  estimate  the  copper-reducing  power  of  the  extracts  with 
Fehling's  solution  upon  a  separate  sample,  and  to  deduct  this 
from  the  total  reducing  power  found  after  treatment  with 
the  starch. 

The  process  is  briefly  as  follows:  A  definite  quantity  (say 
30  mils)  of  gelatinized  starch,  made  from  the  best  Bermuda 

*  Jour.  Chem.  Soc,  XLV  (1884),  i- 


298     THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

arrowroot,  of  3  per  cent  strength,  is  placed  in  a  flask  and 
heated  to  40°  C.  A  weighed  amount  of  the  malt  extract  (say 
20  mils  of  a  I  per  cent  solution)  is  then  added,  and  the  temper- 
ature kept  at  40°  C.  for  exactly  half  an  hour.  At  the  end  of 
this  time  some  sodium  hydroxid  solution  is  added  in  order 
to  check  the  further  action  of  the  diastase  upon  the  starch 
(heating  to  100°  C.  accomplishes  the  same  result).  The  solution 
is  then  diluted  to  a  definite  volume  with  water  and  the  maltose 
produced,  estimated  by  Fehling's  solution  in  the  usual  manner. 
The  quantity  of  reducing  sugar  originally  present  in  the  sample 
must  be  previously  determined,  and  this  amount  deducted 
from  the  total  amount  found  after  treatment  with  starch, 
and  the  remainder  calculated  as  maltose.  Ten  mils  of  Fehling's 
solution  is  the  equivalent  of  0.0806  gm.  of  maltose. 


CHAPTER  XVI 
ESTIMATION  OF  FORMALDEHYDE  * 

The  Ammonia  Method  (Legler).  This  method  is  based 
upon  the  reaction  between  free  ammonia  and  formaldehyde 
in  which  hexamethylene-tetramin  is  formed.  It  is  for  ordinary 
purposes  sufficiently  accurate. 

It  is  this  method  which  is  recommended  by  Lederle  f 
for  use  in  the  laboratory  of  the  New  York  City  Health  Depart- 
ment, and  by  Prescott  in  the  laboratory  of  the  Universtiy  of 
Michigan. 

The  assay  is  conducted  as  follows:    2  mils  of  the  solution 

are  placed  in  a  glass-stoppered  bottle,  the  stopper  of  which 

.    ■  ■  N 

is  thickly  coated  with  petrolatum,  and  50  mils  of  —  ammonia 

solution  added;    let  stand  twelve  hours,  shaking  occasionally. 

N 
Then  determine  the  excess  of  ammonia  by  titrating  with  — 

sulphuric  acid  solution,  using  rosoHc  acid  or  litmus  as 
indicator.  The  excess  of  ammonia  subtracted  from  the 
quantity  added  gives  the  quantity  which  combined  with  the 
formaldehyde,  and  thus  the  amount  of  the  latter  is  ascer- 
tained. 


*  Berichte  d.  Chem.  Ges.,  XVI.,  1335,  1883. 
t  Am.  Drug.,  1897,  246. 

299 


300       THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 
The  reaction  is  represented  as  follows: 

6CH2O     +    4NH3     =     (CH2)6N4     +    6H2O. 

4)180  4)68.04  Hexamethylenetetramin    . 

2)45_  2)17.01  ^ 

22.5  gms.       8.5  gms.  or  1000  mils  —  V.S. 
»  2 

0.0225  gm.  0.0085  gm.  or    i  mils  "     " 

.         N 
Assuming  that  22  mils  of  —  sulphuric  acid  were  employed 

N 
in  the  titration,  22  mils  of  the  —  ammonia  solution  must  have 

2 

been  in  excess,  hence  28  mils  of  the  latter  went  into  combina- 
tion with  the  formaldehyde.  Thus  the  2  mils  of  formaldehyde 
solution  contained  28X0.0225  =  0.63  gm. 

A.  G.  Craig  *  says  that  the  chief  difficulty  in  using  the 
Legler  method  is  the  volatility  of  the  ammonia.  The  difficulty 
is  not  so  much  the  loss  of  strength  in  the  standard  solution, 
but  the  loss  during  the  determination.  He  proposes  the  follow- 
ing scheme  by  which  this  error  is  removed. 

Prepare  a  normal  solution  of  sulphuric  acid.  Make  up  an 
approximately  normal  solution  of  ammonia,  the  exact  strength 
being  immaterial.  Procure  several  three-oimce  prescription 
bottles  with  smooth  sides  and  close-fitting  soft  rubber  stoppers. 
Prepare  a  methyl  orange  solution.  Procure  a  boiler  in  which 
the  bottles  may  be  immersed  to  the  neck  without  upsetting 
(a  large  beaker  will  do).  Take  as  much  of  the  sample  as 
will  contain  0.5  gm.  of  formaldehyde.  Measure  vdth  the 
pipette,  25  mils  of  the  ammonia  solution  into  each  of  the  bottles, 
and  to  half  of  them  add  a  sample  of  formaldehyde;  stopper 
tightly.  If  the  necks  of  the  bottles  are  small,  the  stoppers 
need  not  be  tied  down.  Place  the  bottles  in  the  boiler,  add 
cold  water  to  the  necks,  and  heat  to  boiling.     Boil  for  one 

*  J.  A.  C.  S.,  XXIII,  642  (1901). 


ESTIMATION  OF  FORMALDEHYDE  301 

hour,  and  cool  by  running  in  cold  water  slowly,  being  careful 
not  to  allow  the  cold  water  to  touch  the  hot  bottles.  Titrate 
with  sulphuric  acid  and  methyl  orange  to  the  first  indication 
of  a  color  change.  Take  the  difference  between  the  readings 
for  the  blanks  and  those  for  the  samples,  as  the  ammonia 
consumed  in  normal  mils.  Of  this  difference,  i  mil  =0.0601 
gm.  of  formaldehyde. 

The  Legler  method  is  also  liable  to  error  because  the 
compound  formed  is  a  weak  base,  and  as  such  combines 
with  acid,  while  at  the  same  time  it  is  liable  to  decompose 
into  ammonia  and  formaldehyde  and  thus  give  an  indefinite 
end-point  when  the  residual  ammonia  is  titrated  with  acid. 
Error  is  also  liable  to  be  introduced,  through  the  presence  of 
carbonic  acid  in  the  ammonia  water,  which,  with  the  indicator 
rosolic  acid,  gives  no  sharp  end-reaction. 

The  Ammonium  Chlorid  Method.  In  this  method  a  solu- 
tion of  ammonium  chlorid  is  used,  from  which  ammonium 
is  evolved  by  treatment  with  sodium  hydroxid.  The  excess  of 
alkali  is  then  determined  by  titration  with  standard  solution 
of  sulphuric  acid.  This  method,  as  devised  by  H.  Schiff,* 
and  Modified  by  C.  A.  Male,t  is  as  follows: . 

Introduce  2  gms.  neutral  ammonium  chlorid,  dissolved  in 
20  mils  of  water,  into  a  flask  or  bottle  of  about  200  mils  capacity, 
having  a  well-fitting  stopper.  Dilute  10  mils  of  the  formaldehyde 
solution  to  100  mils  with  water,  and  neutralize  with  sodium 
hydroxid  solution,  as  the  formaldehyde  solution  generally  con- 
tains varying  quantities  of  formic  acid.  Add  20  mils  of  this 
neutralized  solution  to  the  ammonium  chlorid  solution,  then 

N    ■ 
25  mils  of  —  NaOH,  and  immediately  stopper  the  flask,  and 

*  Chem.  Ztg.,  XXVII,  14  (1903). 

t  Pharm.  Jour.,  June,  1905,  844.  See  also  Carl  E.  Smith,  A.  J^  Ph., 
LXX,  86,  (1898). 


302       THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

leave  for  one  hour.     Afterwards  determine  the  excess  of  alkali 

N 
with  —  H2SO4,  using  rosolic  acid  or  litmus  solution  as  indi- 
cator,  both   of   which   give   up   a   sharp   end-reaction.     The 
reaction  and  calculation  is  based  upon  the  following  equation: 

2NH4CI +3CH2O  +  2NaOH = N2  (CH2)3  +  2NaCl  +  5H2O. 

.   N  .         . 

I  mil  —  NaOH  is  equivalent  to  0.045  8^-  ^^  formic  aldehyde. 

This  modified  method  is  quite  as  simple,  and  gives  results 
practically  identical  with  that  of  Romijn. 

Oxidation  by  Hydrogen  Dioxid  (Blank  and  Finkenbeiner)  * 
This  method  depends  upon  the  use  of  hydrogen  dioxid  for 
oxidizing  formaldehyde  into  formic  acid,  in  alkaline  solution. 
The  formic  acid  so  produced  neutralizes  a  portion  of  the  alkali, 
and  the  excess  of  the  latter  is  then  determined  by  titratiui 
with  standard  acid.  The  method  gives  good  results  and  can 
be  very  rapidly  carried  out. 

Three  mils  of  the  formaldehyde  solution  are  placed  into  a  100- 
mil  Erlenmeyer  flask,  the  latter  closed  with  a  well-fitting  stopper, 
and  the  weight  of  the  solution  carefully  taken.  50  mils  of 
normal  NaOH  V.S.  are  then  added,  and  followed  immediately, 
but  slowly,  through  a  small  funnel,  by  50  mils  of  a  3  per  cent 
hydrogen  dioxid  solution,  previously  neutralized  with  normal 
NaOH  V.S.,  using  a  drop  of  litmus  solution.  The  solution  is 
allowed  to  stand  about  15  minutes,  or  until  the  reaction  has 
ceased.  The  funnel  and  the  sides  of  the  vessel  are  then 
rinsed  with  distilled  water,  a  few  drops  of  litmus  solution 
added  and  the  unconsumed  alkali  titrated  with  normal  H2SO4 
V.S.  The  mils  of  the  latter  deducted  from  the  50  mils  of  nor- 
mal NaOH  V.S.  originally  taken  gives  the  quantity  of  the 
*  Berichte  d.  Chem.  Ges.,  XXXI,  2979  (1898);  and  A.  J.  Ph.,  1899,  486. 


ESTIMATION  OF  FORMALDEHYDE  SOS- 

alkali  V.S.  which  was  consumed  in  the  reaction.  The  dif- 
ference multiplied  by  0.03  gm.  gives  the  weight  of  CH2O 
in  the  sample.  To  find  per  cent  multiply  the  result  by  100 
and  divide  by  the  weight  of  sample  taken  for  analysis. 

In  the  original  process,  double  normal  sodium  hydroxid  is 
used.* 

The  reaction  is  as  follows: 

CH2O  +  H2O2  +  NaOH  =  NaCOOH  +  2H2O 

H2O2  oxidizes  formaldehyde  to  formic  acid,  HCOOH,  which 
reacts  with  and  neutralizes  NaOH. 

The  lodometric  Method  (Romijn).t  This  method,  which 
is  considered  the  most  rapid,  most  accurate,  and  most  readily 
applied,  depends  upon  the  fact  that  iodin  in  the  presence  of 
an  alkali  acts  as  an  indirect  oxidizing  agent,  giving,  when 
formaldehyde  is  present,  the  iodid  of  the  base  and  formic  acid: 

CH20  +  l2  +  2NaOH  =  2NaI  +  CHOOH  +  H20. 

As  modified  by  L.  Renter  J  it  is  as  follows: 
Twenty  mils  of  35  to  40  per  cent  forma dlehyde  are  intro- 
duced into  a  graduated  flask  and  distilled  water  added  to  bring 
up  the  volume  to  500  mils.  Of  this  thoroughly  mixed  fluid,  5 
mils  are  introduced  into  a  bottle  capable  of  being  perfectly 
closed.     30  mils  of  normal  sodium  or  potassium  hydroxid  are 

N  .     . 
added,  and  then  —  iodin  solution  allowed  to  flow  in  from 

a  burette  with  constant  agitation,  until  the  fluid  remains 
a  bright  jellow  color  (36-60  mils).  The  shaking  is  then  vig- 
orously continued  for  one  minute,  when  40  mils  of  normal 

*See  C.  Allen  Lyford,  J.  A.  C.  S.,  XXIX,  1227  (1907)  "The  Action  of 
Barium  Peroxid  and  Hydrogen  Peroxid  upon  Formaldehyde." 

fZeitschT.  anal.  Chem.,  XXXVI,  18-24  (1897);  ihid,  XXXIX,  60-63 
(1900). 

X  Ph.  Rev.,  1903,  207. 


304       THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

sulphuric  acid  are  added,  and  then  the  excess  of  iodin  titrated 

N 
with  decinormal  thiosulphate  solution.     Each  mil  of  —  iodin 

solution  consumed  corresponds  to  0.0015  gm.  formaldehyde. 
A  blank  titration  should  always  be  made. 

A  solution  containing  as  much  as  5  per  cent  may  be 
accurately  estimated  by  this  method,  provided  not  more  than 
2  gms.  be  taken.  The  method  is,  however,  not  accurate  in 
the  presence  of  other  aldehydes.  F.  O.  Taylor,*  commenting 
upon  this  method,  says  that  it  is  quite  satisfactory,  but  that 
the  quantities  of  formaldehyde  and  reagents  used  are  unnec- 
essarily large  and  cumbersome,  and  the  method  is  hence  modi- 
fied by  him  as  follows: 

From  a  weighing  bottle,  consisting  of  a  small  Erlenmeyer 
flask,  fitted  with  a  perforated  rubber  stopper  through  which 
passes  a  dropper,  and  containing  about  25  or  30  mils  of  the 
formaldehyde  solution,  weigh  out  accurately  about  10  gms 
of  the  solution  into  a  stoppered  500-mil  flask  and  fill  this  to 
the  mark  with  distilled  water.  For  titration  remove  5  mils  of 
this  solution,  corresponding  to  o.i  gm.  of  the  weighed  quan- 
tity of  formaldehyde,  and  put  into  a  2co-mil  Erlenmeyer  flask. 
Into  another  flask  put  5  mils  of  water  for  a  blank  titration. 
To  both  now  add  20  mils  of  normal  NaOH  and  then  20  mils 

N 
of  an  approximately  —  iodin  solution,  whose  exact  strength 

need  not  be  known,  and  let  stand  for  five  or  ten  minutes  for 

the  entire  completion  of  the  reaction.    Now  add  25  mils  of 

N 
normal  H2SO4  and  titrate  the  excess  of  iodin  with  —  Na2S203. 

10 

The  difference  between  the  mils  of  thiosulphate  used  on  the 

N 
assay  and  the  mils  of  the  blank  is  the  number  of  mils  of  —  iodin 

*  Bull.  Ph.,  Aug.,  1903,  323. 


ESTIMATION  OF  FORMALDEHYDE  305 

N 
consumed   by  the   formaldehyde.     Each  mil   of  —  iodin   so 

used  equals  0.0015  gm.  of  CH2O. 

The  Potassium  Cyanid  Method.*  This  method  is  especially 
applicable  to  solutions  containing  small  quantities  of  formal- 
dehyde. It  depends  upon  the  fact  that  potassium  cyanid  and 
formaldehyde  combine  to  form  an  addition  product,  in  which 
one  molecule  of  potassium  cyanid  combines  with  one  molecule 
of  formaldehyde,  as  shown  by  the  following  equation: 

H 

I 
CH2O  +  KCN  =  N=C- C— O— K. 

I 
H 

In  the  estimation,  the  formaldehyde  is  mixed  with  a  known 

quantity  of  potassium  cyanid   (in  excess),  the  excess  of  the 

latter  being  determined  by  the  use  of  standard  silver  nitr'Bte 

solution,  and  thus  the  quantity  of  potassium  cyanid,  which 

combined   with   formaldehyde   is   found,   and   from   this   the 

quantity  of  formaldehyde  is  calculated. 

N 
The  process  is  carried  out  as  follows  :'\    Ten   mils   of  — 

silver  nitrate  are  treated  with  6  drops  of  50  per  cent  nitric 

acid  in  a  50-mil  flask.     Ten  mils  of  a  solution  of  potassium 

cyanid  (containing  i  gm.  of  KCN  in  500  mils  of  water)  are 

then  added  and  well  shaken.     An  aliquot  part  of  the  filtrate, 

N. 
say  25   mils,   is  then  titrated  by  Volhard's  method  with  — 

ammonium  sulphocyanate  for  excess  of  silver. 

Another  10  mils  of  silver  nitrate  solution  is  then  acidified 

*  Romijn,  Zeitschr.  anal.  Chem.,  XXXVI,  18-24  (1897). 
t  Bernard  H.  Smith,  J.  A.  C.  S.,  XXV,  1032  (1903). 


306      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

with  nitric  acid  and  treated  with  lo  mils  of  the  potassium 

cyanid  solution  to  which  has  been  added  a  measured  quantity 

of  dilute  formaldehyde  solution.     The  whole  is  made  up  to 

50  mils  and  then  filtere.''.     25  mils  of  the  filtrate  are  titrated 

N 
with   —    ammonium    sulphocyanate    for   excess   of   silver   as 

before. 

The  difference  between   these  two  results  multiplied   by 

2  gives  the  amount  of  potassium  cyanid  which  was  used  by 

N   ■ 
the  formaldehyde,  in  terms  of  —  sulphocyanate. 

N 
Each  mil  of  —  V.S.  represents  0.003  g^^-  of  formaldehyde. 

The  best  results  by  this  method  are  obtained,  if  the  solu- 
tion of  formaldehyde  is  diluted  to  below  i  per  cent.  With 
I  per  cent  solutions  it  is  necessary  to  use  15  mils  of  the  silver 
solution.  In  estimating  very  dilute  solutions,  it  is  advisable 
to  use  a  200-mil  flask  and  to  take  100  mils  of  the  filtrate  for 
the  titration.  It  is  possible  by  this  method  to  determine 
with  accuracy  one  part  in  100,000. 

Paraformaldehyde  (CH20)3.  This  is  a  polymeric  form  of 
formaldehyde  and  may  be  assayed  by  the  same  methods.  The 
U.S.P.  IX  recommends  the  process  in  which  hydrogen  dioxid 
is  used,  the  procedure  being  practically  the  same. 

Each  mil  of  normal  alkali  hydroxid  V.S.  corresponds  to 
0.03  gm.  of  (CH20)3. 

Acetone  (Dimethyl-ketone)  (CH3. CO. CH3  =  58.05).  This 
may  be  assayed  by  the  iodometric  method,  used  for  formalde- 
hyde. One  mil  of  acetone  is  accurately  weighed  in  a  stoppered 
weighing-fiask  tared  together  with  some  distilled  water.  This  is 
transferred  to  a  looo-mil  flask  and  diluted  to  the  mark  with 
distilled  water.  Twenty-five  mils  of  normal  potassium  hydroxid 
V.S.  are  placed  in  a  250-mil  glass-stoppered  flask  and  to  this 


ESTIMATION.  OF  FORMALDEHYDE  307 

is  added  exactly  25  -mils  of  the  acetone  solution  (representing 
one-fortieth  of  the  weight  of  acetone  taken),  then  with  constant 
agitation  of  the  flask,  35  mils  of  decinormal  iodin  V.S.  is 
added,  and  the  mixture  allowed  to  stand  for  fifteen  minutes. 
At  the  end  of  this  time  26  mils  of  normal  hydrochloric  acid 
V.S.  is  added  and  the  titration  begun  at  once,  with  decinormal 
sodium  thiosulphate  V.S.,  using  starch  as  indicator. 

A  blank  test  is  conducted  with  the  same  quantities  of  re- 
agents. The  difference  in  the  quantity  of  decinormal  iodin 
V.S.  consumed  in  the  blank  test  and  that  in  the  assay,  repre- 
sents the  quantity  taken  up  by  the  acetone.  " 

Each  mil  of  this  difference  corresponds  to  0.0009675  gm.  of 
acetone.     The  reaction  is 

CH3.CO.CH3+*6I  +  4KOH  =  CHl3  +  KC2H302  +  3KI  +  3H20. 


CHAPTER  XVII 

ESTIMATION  OF  ALCOHOL  IN  TINCTURES  AND 
BEVERAGES 

The  quantity  of  alcohol  contained  in  dilute  spirit,  which 
leaves  no  residue  upon  evaporation,  may  be  ascertained  by 
taking  the  sp.gr.  and  referring  to  the  alcohol  table.  When 
taking  the  specific  gravity,  the  temperature  of  the  liquid 
should  be  15!°  C.  (60°  R). 

In  Wines,  Beer,  Tinctures  and  other  alcoholic  liquids 
containing  vegetable  matter,  the  sp.gr.  of  thc/sample  is  taken 
at  15!°  C.  (60°  F.)  and  noted.  A  certain  quantity  (say 
100  mils)  is  measured  off  and  evaporated  to  one  half,  or  until 
all  odor  of  alcohol  has  passed  off,  the  evaporation  being  con- 
ducted without  ebullition,  in  order  that  particles  of  the  material 
may  not  be  carried  off  by  the  steam.  The  liquid  left  is 
then  diluted  with  distilled  water,  cooled  to  60°  F.  and  made 
up  to  the  original  volume  (100  mils),  and  the  sp.gr.  taken. 
Lastly,  we  calculate:  the  sp.gr.  before  evaporating  is  divided 
by  the  sp.gr.  after  evaporating,  and  the  quotient  will  be  the 
sp.gr.  of  the  water  and  alcohol  only  of  the  liquor.  Then 
by  referring  to  the  alcohol  table  the  percentage  of  alcohol 
contained  in  the  liquor  is  obtained. 

Example.  The  liquor  before  evaporating  had  a  sp.gr. 
of  0.9951;  after  evaporation  and  dilution  to  100  mils  the  sp.gr. 
was  found  to  be  1.0081. 

=0.987,  the  sp.gr.  of  the  contained  spirit. 


1.0081 

308 


ALCOHOL  IN  TINCTURES  AND  BEVERAGES 


309 


TABLE  FOR  ASCERTAINING  THE  PERCENTAGES  RESPECTIVELY 
OF  ALCOHOL  BY  WEIGHT,  BY  VOLUME,  AND  AS  PROOF 
SPIRIT,  FROM  THE  SPECIFIC  GRAVITY. 

Condensed  from  the  excellent  Alcohol  Tables  of  Mr.  Hehner  in  the 
"  Analyst,"  vol.  v.  pp.  43-63. 


Specific 
Gravity 

is-s°. 

Absolute 

Alcohol 

by    vv'ght. 

Per  cent. 

Absolute 

Alcohol 

by  vol'me. 

Per  cent. 

Proof          S 
Spirit.          G 
Per  cent. 

pecific 
ravity 
5-S^. 

Absolute 

Alcohol 

by  w'ght. 

Per  cent. 

Absolute 

Alcohol 

by  vol'me. 

Per  cent. 

Proof 

Spirit. 

Per  cent. 

I .0000 

0.00 

0.00 

0.00 

9489 

35-05 

41-90 

73.43 

•9999 

0.05 

0.07 

0. 12 

9d79 

35-55 

42.45 

74.39 

.9989 

0.58 

0.73 

1.28 

9469 

36.06 

43-01 

75.37 

.9979 

1 .  12 

1.42 

2.48 

9459 

36.61 

43   63 

76.45 

.9969 

1-75 

2.20 

3-85 

9449 

37-17 

44-24 

^rp 

•9959 

2.33 

2-93 

5-13 

9439 

H-'^l 

44-86 

78.61 

.9949 

2.89 

3-62 

6-34 

9429 

38.28 

45-47 

79.68 

.99.39 

3-47 

4-34 

7.61 

9419 

38.83 

46.08 

80.7s 

•9929 

4.06 

5-08 

8.90 

9409 

39-35 

46.64 

81.74 

.9919 

4.69 

5.86 

10.26 

9399 

39-85 

47-18 

82.69 

.9909 

5-31 

6.63 

II  .62 

9389 

40.35 

47.72 

83.64 

.9899 

5-94 

7.40 

12.97 

9379 

40.8s 

48.26 

84.58 

.9889 

6.64 

8.27 

14.50 

9369 

41.35 

48.80 

?!•" 

.9879 

7-33 

9-13 

15-99 

9359 

41.85 

49-34 

86.47 

.9869 

8.00 

9-95 

17-43 

9349 

42.33 

49-86 

87.37 

.9859 

8.71 

10.82 

18.96 

9339 

42.81 

SO-37 

88.26 

.9849 

9-43 

II .  70 

20.50 

9329 

43-29 

50-87 

89.15 

.9839 

10.15 

12.58 

22.06 

9319 

43-76 

SI. 38 

90.03 

.9829 

10.92 

13-52 

23-70 

9309 

44.23 

.    Si-87 

90.89 

.9819 

II  .69 

14.46 

25-34 

9299 

44.68 

52.34 

91.73 

.9809 

12.46 

15.40 

26.99 

9289 

45.14 

52.82 

92.56 

.9799 

13-23 

16.33 

28.62 

9279 

45-59 

53-29 

93-39 

.9789 

14.00 

17.26 

30.26 

9269 

46.05 

53-77 

94.22 

.9779 

14-91 

18.36 

32.19 

9259 

46.50 

54-24 

95.05 

.9769 

15-75 

19-39 

33-96 

9249 

46.96 

54-71 

95.88 

.9759 

16.54 

20.33 

35-63 

9239 

47.41 

55-18 

96.70 

.9749 

17-33 

21  .29 

37-30 

9229 

47.86 

55-65 

97^52 

.9739 

18.15 

22.27 

39   03 

9219 

48.32 

56.  II 

98.34 

.9729 

18.92 

23.10 

40.64 

9209 

48.77 

56.58 

99-16 

.9719 

19-75 
20-58 

24.18 
2517 

42.38, 

9199 

49.20 

57-02 

99   03 

.9709 

44-12 

.9699 

21.38 

26.13 

45  -  79 

9198 

49  -  24 

57.06 

loo.oaPs 

.9689 

22.15 

27.04 

47-39 

.9679 
.9669 

22.92 
23    69 

27-95 
28.86 

48.98       

50.57 

9189 

49-68 

57-49 

100. 76 

.9659 

24.46 

29.  76 

52.16 

9179 

50.13 

57-97 

101.59 

.9649 

25-21 

30.65 

53-71 

9169 

50.57 

58-41 

102.35 

•9639 

25-93 

31-48 

55-18 

9159 

51-00 

58.8s 

103.12 

.9629 

26.60 

32-27 

56.55 

9149 

51.42 

59-26 

103.8s 

.9619 

27.29 

3306 

57-94 

-9139 

51-83 

59-68 

104-58 

.9609 

28.00 

33    89 

59-40 

9129 

52-27 

60. 12 

105.3s 

•9599 

28.62 

34-61 

60.66 

.9119 

52.73 

60.  56 

106.15 

■9589 

29.27 

35-35 

61.95 

.9109 

53-17 

61  .02 

106.93 

•9579 

29-93 

36.12 

63-30 

-9099 

S3.6i 

61.45 

107.69 

.9569 

30-50 

36.76 

64-43 

.9089 

S4-OS 

61.88 

108.45 

.9559 

31-06 

37-41 

65.5s 

-9079 

54-52 

62.36 

109.28 

•9549 

31.69 

38-11 

66.80 

.9069 

55-00 

62.84 

110.12 

•  9539 

32.31 

38.82 

68.04 

-9059 

55-45 

63-28 

110.92 

.9529 

32-94 

39-54 

69.29 

-9049 

55-91 

63-73 

111.71 

.9519 

33-53 

40.  20 

70.46 

-9039 

56.36 

64-18 

112.49 

•  9509 

34.10 

40.84 

71-58 

.9029 

56.82 

64-63 

113.26 

.9499 

34-57 

41-37 

72.50 

.9019 

57-25 

65-05 

113.99 

310      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 


Specific 

Gravity 

15-5°. 

Absolute 
Alcohol 
by  w'ght. 
Per  cent. 

Absolute 

Alcohol 

by  vol'me. 

Per  cent. 

Proof 

Spirit. 

Per  cent. 

Specific 

Gravity 

15.5°. 

Absolute 

Alcohol 

by    w'ght. 

Per  cent. 

Absolute 

Alcohol 

by  vol'me. 

Per  cent. 

Proof 

Spirit. 

Per  cent. 

.9009 

57-67 

65.45 

114.69 

.8429 

82.19 

87.27 

152.95 

.8999 

58.09 

65.85 

115. 41 

.8419 

82.58 

87.58 
87.88 

153.48 

.8989 

58.55 

66.29 

116. 18 

.8409 

82.96 

iS4-ot 

.8979 

59  00 

66.74 

116.96 

.8399 

83.35 

88.19 

154-54 

.89^9 

59-43 

67.15 

117.68 

.8389 

83.73 

88.49 

155-07 

.8959 

59.87 

67-57 

118. 41 

.8379 

84.12 

88.79 

155.61 

.8949 

60.29 

67.97 

119. 12 

.8369 

84.52 

89.11 

156.16 

.8939 

60.71 

68.36 

119.80 

.8359 

84.92 

89.42 

156.71 

.8929 

61.13 

68.76 

120.49 

.8349 

85.31 

89.72 

157.24 

.8919 

61.54 

69.15 

121. 18 

.8339 

85.69 

.    90.02 

157.76 

61.96 

69.54 

121.86 

.8329 

86.08 

90.32 

158.28 

.8899 

62 .  41 

69.96 

122. 6i 

.8319 

86.46 

90.61 

158.79 

.8889 

62.86 

70.40 

123.36 

.8309 

86.8s 

90.90 

159.31 

,8879 

63.30 

70.81 

124.09 

.8299 

87.23 

91 .20 

159.82 

.8869 

63.74 

71.22 

124.80 

.8289 

87.62 

91.49 

160.33 

.8859 

64.17 

71.62 

125.51 

.8279 

88.00 

91.78 

160.84 

.8849 

64.61 

72.02 

126.22 

.8269 

88.40 

92.08 

161.37 

.8839 

65.04 

72.42 

126.92 

.8259 

88.80 

92.39 

161 .gi 

.8829 

65.46 

72.80 

127.59 

.8249 

89.19 

92.68 

162.43 

.8819 

65.88 

73.19 

128.25 

.8239 

89.58 

92.97 

162.93 

.8609 

66.30 

73-57 

128.94 

.8229 

89.96 

93.26 

163.43 

.8799 

66.74 

73.97 

129.64 

.8219 

90.32 

93    52 

163.88 

.8789 

67-17 

74.37 

130.33 
130.98 

.8209 

90.68 

93.77 

164.33 

.8779 

67.58 

74.74 

.8199 

91.04 

94.03 

164.78 

.8769 

68.00 

75.12 

131.64 

.8189 

91.39 

94.28 

165.23 

.8759 

68.42 

75-49 

132.30 

.8179 

91.75 

94.53 

165.67 

.8749 

68.83 

75-87 

132.9s 

.8169 

92.  II 

94-79 

166.12 

•  8739 

69.25 

76.24 

133.60 

.8159 

92.48 

95.06 

166.58 

.8729 

69.67 

76.61 

134.25 

.8149 

92.85 

95.32 

167.04 

.8719 

70.08 

76.98 

134.90 

.8139 

93-22 

95.58 

167.50 

.8709 

70.48 

77-32 

135.51 

,8129 

93  -  59 

98.54 

167.96 

.8699 

70.88 

77.67 

136.13 

.8119 

93   96 

96.11 

168.24 

.8689 

71.29 

78.04 

136.76 

.8109 

94.31 

96.34 

168.84 

.8679 

71.71 

78.40 

137.40 

.8099 

94  66 

96.57 

169.24 

.8669 

72.13 

78.77 

138.05 

.8089 

9S-00 

96,80 

169.65 

.8659 

72.57 

79.16 

138.72 

.8079 

95-36 

97.05 

170.07 

.8649 

73- 00 

79-54 

139.39 

.8069 

95-71 

97.29 

170.50 

.8639 

73-42 

79-90 

140.02 

.8059 

96.07 

97.53 

170.99 

.8629 

73.83 

80.26 

140.65 

.8049 

96.40 

97.75 

171.30 

.8619 

74.27 

80.64 

141.33 

.8039 

96.73 

97.96 

171.68 

.8609 

74-73 

81.04 

142.03 

#8029 

97.07 

98.18 

172.05 

.8599 

75-18 

81.44 

142.73 

.8019 

97.40 

98.39 

172.43 

.8589 

75-64 

81.84 

143.42 

.8009 

97.73 

98.61 

172.80 

•  8579 

76.08 

82.23 

144.10 

.7999 

98.06 

98.82 

173.17 

.8569 

76.50 

82.58 

144.72 

.7989 

98.37 

99   00 

173.50 

•  8559 

76.92 

82.93 

145.34 

.7979 

98.69 

99.18 

173.84 

-8549 

77.33 

83.28 

145.96  • 

.7969 

99.00 

99.37 

174.17 

-8S35 

11:11 

Pt 

146.57 

-7959 

99.32 

99.57 

174.52 

-8529 

147.17 

.7949 

99.6s 

99-77 

174.87 

.8519 

78.56 

84-31 

147.75 

.7939 

99-97 

99  98 

175.22 

.8509 

78.96 
79.36 

84.64 
84.97 

148.32 
148.90 

-8499 

.8489 

79.76 
80.17 
80.58 

85.29 

149.44 

.8479 

85.63 

150.06 

Absolute 

Alcohol. 

.8469 

85.97 

150.67     1 

.8459 
.8449 

81 .00 

86.32 

151.27 

81.40 

86.64 

151.83 

.8439 

81.80 

86.96 

152.46 

.  7938 

100.00 

100 . 00 

175.25 

ALCOHOL  IN  TINCTURES  AND  BEVERAGES        311 

Then  by  referring  to  the  table  we  find  that  this  sp.gr.  cor- 
responds to  7.33  per  cent,  by  weight,  of  absolute  alcohol. 

Another  Way  is  to  boil  the  liquid  in  a  retort,  condense 
the  vapor,  and  when  all  the  alcohol  has  passed  over  add 
sufiicient  water  to  the  distillate  to  make  up  the  original  vol- 
ume, at  the  temperature  of  15!°  C.  (60°  F.).  Then,  by 
taking  the  sp.gr.  of  this  diluted  distillate,  the  quantity  of 
absolute  alcohol  is  found  by  reference  to  the  table.  This 
latter  method  requires  the  taking  of  the  sp.gr.  but  once  and 
gives  more  accurate  results. 

The  details  of  this  method  are  as  follows:  Place-  25  mils 
of  the  liquid  measured  at  any  definite  temperature  between 
15°  and  30°  C.  in  a  distilling  flask  of  about  200  mils  capacity 
and  connected  with  a  suitable  condenser;  add  50  mils  of  dis- 
tilled water,  distil  into  a  50-mil  graduated  flask  at  such  a  rate 
that  about  48  mils  of  distillate  will  be  received  in  a  half  hour. 
Dilute  with  water  of  the  same  temperature  to  50  mils.  Take 
the  specific  gravity  at  any  definite  temperature  between  15° 
and  30°  C.  by  means  of  a  pycnometer,  and  refer  to  the  alcohol- 
ometric  table  for  per  cent  of  absolute  alcohol  and  multiply 
by  the  dilution  factor  to  find  the  per  cent  of  alcohol  in  the 
original  liquid. 

If  the  preparation  is  suspected  to  contain  less  than  25  per 
cent  of  alcohol,  take  50  mils  for  analysis;  if  it  is  suspected  to 
contain  over  50  per  cent,  take  12.5  mils  for  analysis.  In  either 
case,  the  volume  of  the  distillate  should  be  made  up  to  50 
mils. 

If  the  preparation  contains  iodin,  conbine  it  with  zinc  dust, 
or  add  sufficient  sodium  thiosulphate  to  take  up  the  iodin. 
In  the  latter  case  add  a  few  drops  of  sodium  hydroxid  T.S. 
to  prevent  sulphur  from  distilling  over.  If  volatile  acids  are 
present,  neutralize  with  sodium  hydroxid  T.S.  If  volatile 
bases  are  present  neutralize  with  diluted  sulphuric.     If  both 


312      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

volatile  acids  and  volatile  bases  are  present,  neutralize  first 
with  dilute  sulphuric  acid  and  distil  about  50  mils,  then  neu- 
tralize the  distillate  with  sodium  hydroxid  T.S.  and  again  distil 
to  obtain  50  mils  of  distillate. 

If  acetone,  camphor,  chloroform,  ether,  glycerin  (30  per 
cent  or  over)  volatile  oils  or  other  volatile  products  are  sus- 
pected to  be  present,  transfer  the  first  distillate  to  a  separator, 
saturate  it  with  sodium  chlorid,  add  15  mils  of  petroleum 
benzin  (boiling  point  40-50°  C.)  and  shake  for  one  or  two 
minutes.  After  the  liquids  have  completely  separated,  draw 
off  the  lower  alcoholic  salt  solution  into  a  second  separator 
and  repeat  the  extraction  with  15  mils  of  petroleum  benzin. 
Again  draw  of  the  lower  alcoholic  salt  solution.  Introduce 
this  into  a  200-mil  distilling  flask.  Wash  the  combined  benzin 
solutions  with  about  25  mils  of  saturated  sodium  chlorid  solu- 
tion and  add  the  washings  to  the  distilling  flask,  and  distil 
about  45  or  48  mils,  bring  to  the  original  temperature,  dilute 
with  water  of  the  same  temperature  to  50  mils,  determine 
specific  gravity  and  alcohol  per  cent  as  directed  above. 


PART  III 

A  FEW  GASOMETRIC  ANALYSES 


CHAPTER  XVIII 


THE  NITROMETER 


For  general  gas  analysis,  and  for  the  rapid  estimation  of 
such  substances  as  ethyl  nitrite,  hydrogen  dioxid,  urea,  bleaching 
powder,  manganese  dioxid,  etc.,  an  instrument  called  the 
nitrometer  is  used. 

The  apparatus  in  its  simplest  form  is  shown  in  Fig.  57. 
It  consists  of  a  measuring  tube,  a,  of  50-  or 
loo-mils  capacity,  and  graduated  in  tenths 
of  a  mil.  This  is  connected  by  means  of  a 
stout  rubber  tube  with  an  open  equilibrium 
tube,  h,  also  called  ''control-tube,"  "pres- 
sure-tube," or  "  level-tube."  Both  tubes  are 
preferably  provided  with  a  globular  expan- 
sion near  the  lower  end,  and  are  held  by 
suitable  clamps  upon  a  stand,  in  such  a 
manner  that  either  tube  may  be  readily  and 
quickly  clamped  at  a  higher  or  lower  level. 
The  measuring  tube  is  fitted  at  the  top 
with  a  stop-cock,  c,  and  a  graduated  glass 
tube  or  cup,  d.  Some  nitrometers  are  pro- 
vided with  a  three-way  stop-cock,  so 
arranged  that  according  to  the  way  it  is 
turned,  it  will  discharge  the  contents  of  the 
cup   either   into    the  measuring   tube   below,  or  out  into  the 

313 


Fig.  57. 


314       THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

waste  opening  which  is  usually  placed  at  e,  or  it  will  discharge 
the  contents  of  the  measuring  tube  into  the  waste  opening. 

With  this  apparatus  gases  can  be  rapidly  and  accurately 
measured  at  definite  temperature  and  pressure. 

In  measuring  the  gas  the  instrument  is  filled  with  some 
liquid  in  which  the  gas  is  insoluble — generally  mercury.  In 
many  cases  a  saturated  solution  or  salt  may  be  used. 

Suppose  we  fill  the  instrument  with  mercury  in  such  quan- 
tity that  when  the  stop-cock  is  opened  and  the  control-tube 
raised,  the  mercury  will  rise  as  far  as  the  top,  and  about  two 
inches  in  the  control-tube. 

The  top  is  now  closed,  the  control-tube  lowered,  and  a 
little  carbonic  acid  gas  admitted  through  e.  The  top  is 
then  again  closed,  and  the  instrument  allowed  to  stand  until 
its  contents  have  acquired  the  temperature  of  the  room.  A 
centigrade  thermometer  suspended  to  the  stand  will  then  give 
the  temperature  of  the  gas. 

The  control-tube  is  now  raised  or  lowered  so  as  to  make 
the  level  of  the  liquid  in  both  tubes  the  same.  This  makes 
the  pressure  in  the  tube  the  same  as  the  atmospheric  pressure 
outside,  and,  by  referring  to  a  barometer  standing  near,  this 
pressure  is  ascertained. 

We  now  have  a  definite  volume  of  the  gas  at  a  known 
temperature  and  pressure. 

It  now  only  remains  to  read  off  the  volume  of  the  gas, 
and  correct  it  to  the  normal  temperature  and  pressure  by 
Charles'  and  Boyle's  laws,  respectively. 

The  normal  temperature  and  pressure  is  o°  C.  and  760 
mm.  pressure.  Although  the  U.S.P.  adopts  a  standard  tem- 
perature of  25°  C. 

The  weight  of  the  gas  in  grams  may  then  be  calculated  from  its 
volume  by  multiplying  the  number  of  mils  at  the  normal  tempera- 
ture and  pressure,  by  the  weight  of  i  mil  of  the  gas  in  grams 


THE  NITROMETER  315 

This  weight  may  be  found  as  follows: 

looo  mils  of  hydrogen  at  normal  temperature  and  pres- 
sure weigh  C.C896  gm.  One  mil  of  H  then  weighs  0.0C00896 
gm. 

One  mil  of  oxygen  weighs  sixteen  times  as  much,  and 
I  mil  of  nitrogen  weighs  fourteen  times  as  much.  There- 
fore, by  multiplying  the  weight  of  i  mil  of  H  by  the 
atomic  weight  of  an  elementary  gas,  or  half  the  molecular 
weight  of  a  compound  gas,  the  weight  of  i  mil  of  that  gas  is 
obtained. 

According  to  the  Law  of  Charles,  the  volume  of  a  gas 
under  constant  pressure  varies  directly  with  the  absolute  tem- 
perature. 

All  gases  expand  or  contract  by  ^|^  of  their  volume  for 
each  centigrade  degree  of  temperature  increased  or  decreased. 

We  may  regard  a  gas  at  0°  C.  as  having  passed  through 
273°  C.  In  other  words,  273°  below  zero  must  be  regarded 
as  the  absolute  zero,  and  0°  C.  as  273°  absolute  temperature. 

Thus  the  absolute  temperature  centigrade  is  the  observed 
temperature  +  273°. 

Example.  A  given  volume  of  oxygen  gas  at  15°  C.  measures 
20  mils.     What  will  it  measure  at  0°  C.  ? 

o°  +  273°X2o  273°  X  20       ^  .,         . 

X5°;%3°       ""     -^^88^  =  ^8.95  mils.    Ans. 

Boyle's  Law.  The  volume  of  a  confined  gas  is  inversely 
proportional  to  the  pressure  brought  to  bear  upon  it.  That 
is,  the  less  the  pressure,  the  greater  the  volume,  and  vice 
versa. 

Rule.  Multiply  the  observed  volume  by  the  observed  pres- 
sure, and  divide  by  the  normal  pressure. 

Example.    A  given  volume    of  gas  at  7*50  mm.  pressure 


316      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

measures  20  mils.     What  will  it  measure  at  760  mm.  (the  nor- 
mal pressure)  ? 

750X20  mils 

7- =19.73  mils.     Ans. 

Now  let  us  take  an  example  in  which  both  laws  are  involved. 

A  given  volume  of  oxygen  at  15°  C.  subjected  to  a  pressure 
of  750  mm.  measures  20  mils.  What  will  it  measure  at  the 
normal  temperature  and  pressure?— i.e.,  0°  C.  and  760  mm. 

In  the  first  example  we  find  that  20  mils  of  oxygen  at  15'' 
C.  will  measure  at  0°  C.  18.95  ^i^^-     Then 

750X18.95  mils 


760 


=  18.70  mils.     Ans. 


Now  to  find  the  weight  of  this  volume  of  oxygen,  we  pro- 
ceed as  follows : 

I  mil  of  H  weighs  0.0000896  gm.; 

I  mil  of  O  weighs  16X0.0000896  =  0.0014336  gm. ; 

18.70  mils  of  0  =  18.70X0.0014336  gm.,  or  0.02680832  gm. 

In  the  U.S. P.  the  standard  for  temperature  and  pressure 
is  25°  C.  and  760  mm.  and  hence  all  pharmaceutical  assays 
should  be  made  in  accordance  with  this  standard,  and  all  gases 
measured  at  this  temperature  and  pressure,  or  corrections  made 
by  calculation. 

Example.  A  volume  of  gas  measures  20  mils  at  15°  C. 
What  will  it  measure  at  25°  C.  ? 

25°  +  273°  X  20  298X20 

■^ — 5-7-^=^ — 5—     or     r-— =  20.7  mils. 

15° +  273°  288  ^ 

The  following  tables  from  the  United  States  Pharma- 
copoeia IX  will  be  found  very  useful  for  making  temperature 
and  pressure  corrections. 


THE  NITROMETER 


317 


FACTORS  FOR  TEMPERATURE  CORRECTIONS 

(Normal  Temperature,  15°  C.) 


Temperature. 

Factor. 

Temperature. 

Factor. 

Temperature. 

Factor. 

15°  c. 

16°  c. 

17°  c. 
18°  c. 
19°  c. 
20°  c. 

21°  C. 

1 .035 
1. 031 
1.028 
1.024 
1. 021 
1. 017 
1. 014 

22°  C. 

23°  c. 
24°  c. 
25°  C. 
26°  C. 
27°  C. 
28°  C. 

I.OIO 
1.007 
1.003 
1. 000 
0.997 

0-993 
0.990 

29°  C. 
30°  c. 
31°  c. 
32°  c. 
33°  C. 
34°  C. 
35°  C. 

0.987 
0.983 
o.qSo 
0.977 
0.974 
0.971 
0.968 

Example.  Assuming  that  the  volume  of  a  gas  read  off 
was  41  mils  at  30°  C.  and  it  is  desired  to  ascertain  the  corre- 
sponding volume  at  25°  C,  then  the  41  mils  must  be  multiplied 
by  0.983.  The  result  will  be  40.30  mils  as  the  equivalent  volume 
of  gas  at  25°  C. 

FACTORS  FOR  CORRECTION  FOR  BAROMETRIC  PRESSURE 

(Normal  Barometer,  760  mm.) 


Barometeb 

.  Reading. 

Barometer  Reading. 

Factor. 

Factor. 

Millimeters. 

Inches. 

Millimeters. 

Inches. 

790 

31.10 

1.039 

660 

25.98 

0.868 

780 

30.71 

1.026 

650 

25.59 

0.855 

770 

3031 

1. 013 

640 

25.20 

0.842 

760 

29.92 

1. 000 

630 

24.80 

0.829 

750 

29.53 

0.987 

620 

24.41 

0.816 

740 

29.13 

0.974 

610 

24.02 

0.803 

730 

28.74 

0.961 

600 

23.62 

0.789 

720 

28.35 

0.947 

590 

23:23 

0.776 

710 

27.95 

0.934 

580 

22.83 

0.763 

700 

27.56 

0.921 

570 

22.44 

0.750 

690 

27.17 

0.908 

560 

22.05 

0.737 

680 

26.77 

0.895 

550 

21.65 

0.724 

670 

26.38 

0.882 

540 

21.26 

0.711 

Example.  Assuming  that  the  volume  of  gas  read  off  was 
41  mils  at  590  mm.  barometric  pressure,  and  it  is  desired  to 
ascertain  the  corresponding  volume  at  normal  pressure  (760 
mm.),  then  the  41  mils  must  be  multiplied  by  0.776.  The 
result  will  be  31.81  mils. 


CHAPTER  XIX 
ASSAY  OF  NITRITES 

Spirit  of  Nitrous  Ether.  This  is  an  alcoholic  solution  of 
ethyl  nitrite  (C2H5N02=  75.05),  yielding,  when  freshly  pre- 
pared and  tested  in  the  nitrometer,  not  less  than  11  times  its 
own  volume  of  nitrogen  dioxid  (NO  =  30.01). 

When  nitrites  are  mixed  with  an  excess  of  KI  and  acid- 
ulated with  H2SO4,  iodin  is  liberated,  and  all  the  nitrogen 
of  the  nitrite  is  evolved  in  the  form  of  NO,  as  shown  in  the 
equation 

2C2H5N02  +  2KI  +  2H2S04=2C2H50H  +  2KHS04  +  l2  +  2NO. 

150. 1  60.02 

The  process  is  conducted  as  follows: 

Open  the  stop-coc"k  of  the  measuring  tube,  raise  the  control- 
tube,  and  pour  into  the  latter  a  saturated  solution  of  NaCl 
until  the  measuring  tube,  including  the  bore  of  the  stop-cock, 
is  completely  filled.  Then  close  the  stop-cock  and  fix  the 
control-tube  at  a  lower  level.  Now  introduce  into  the  funnel 
at  the  top  of  the  measuring  tube  a  weighed  quantity  (about 
4  gms.)  *  of  spirit  of  nitrous  ether;  open  the  stop-cock  and 
allow  the  spirit  to  run  into  the  nitrometer,  being  careful  that 

*  It  is  conveninent  to  take  5  mils  accurately  measured,  and  calculate  its 
weight  by  multiplying  by  the  specific  gravity,  but  better  to  take  about  40 
mils  of  the  spirit,  weigh  it  accurately,  and  then  add  sufficient  alcohol  to  make 
exactly  100  mils.  This  is  mixed  thoroughly  and  10  mils  of  the  solution  taken 
for  analysis. 

318 


ASSAY  OF  NITRITES  319 

no  air  enters  at  the  same  time.  Ten  mils  of  potassium  iodid 
T.S.  are  now  added  in  the  same  manner,  and  followed  by 
lo  mils  of  normal  sulphuric  acid  V.S.  Effervescence  takes 
place  immediately,  and  after  thirty  to  sixty  minutes,  when 
the  volume  of  gas  has  become  constant,  the  control-tube  is 
lowered  so  as  to  make  the  level  of  the  liquid  in  both  tubes 
the  same,  and  the  volume  of  the  gas  in  the  graduated  tube 
read  off. 

This  volume,  multiplied  by  0.00307  gm.,  gives  the  weight 
of  ethyl  nitrite  in  the  spirit  taken  for  analysis.  The  product 
multiplied  by  loo,  and  then  divided  by  the  weight  of  the  spirit 
taken,  gives  the  per  cent  of  pure  ethyl  nitrite  present. 

The  temperature  correction  is  one-third  of  one  per  cent 
of  the  total  percentage  found,  for  each  degree,  additive  if 
the  temperature  is  below,  subtractive  if  above  25°  C.  The 
barometric  correction  is  :fo  of  one  per  cent  for  each  millimeter, 
additive  if  above,  subtractive  if  below,  760. 

The  volume  of  NO  generated  at  the  ordinary  indoor  tem- 
perature (assumed  to  be  at  or  near  25°  C,  77°  F.)  should 
not  be  less  than  55  mils  if  5  mils  of  the  spirit  are  taken,  corre- 
sponding to  about  4  per  cent  of  pure  ethyl  nitrite. 

Sodium  chlorid  solution  is  used  in  the  above  assay,  because, 
owing  to  its  density,  the  spirit  will  float  on  top,  and  the  gas 
evolved  will  not  dissolve  in  it.  At  the  same  time  the  expense 
of  using  mercury  is  saved.  It  is  important  that  no  air  be 
allowed  to  get  into  the  measuring  tube,  because  this  would 
convert  the  NO  into  a  higher  oxid  of  nitrogen,  which  would 
dissolve  in  the  salt  solution,  and  thus  vitiate  the  result. 

Example.  Four  gms.  of  spirit  of  nitrous  ether  (sp.gr. 
0.823)  are  treated  in  a  nitrometer,  and  the  NO  evolved  measures 
55  inils. 

The  temperature  at  which  the  operation  is  conducted  is 
25°  C,  and  the  atmospheric  pressure  normal. 


320      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 
What  per  cent  of  ethyl  nitrite  is  present  in  the  sample  ? 

0.00^07  XS'^X  TOO 

^-^ — — =3.'72  per  cent. 

Amyl  Nitrite  is  a  liquid  containing  about  80  per  cent  of 
amyl  nitrite  (principally  iso-amyl  nitrite),  C5HiiN02=  117.09, 
together  with  variable  quantities  of  undetermined  compounds. 

The  assay  is  as  follows:    Take  about  3  mils  of  the  amyl 

nitrite,   which  has  been  previously  shaken  with  0.5   gm.   of 

potassium  bicarbonate  and   carefully  decanted.     Put  into  a 

tared  loo-mil  measuring  flask,  and  weigh  it  accurately;    add 

alcohol  to  bring  the  volume   to  exactly  loo  mils.     Ten  mils 

of  this  alcoholic  solution  are  introduced  into  the  nitrometer 

as  directed  for  spirit  of  nitrous  ether;    10  mils  of  potassium 

N 
iodid  solution   (20  per  cent)  and  10  mils  of  —    H2SO4  V.S. 

are  then  added,  and  the  volume  of  NO  generated,  measured 
at  the  ordinary  indoor  temperature  (25°  C.  or  77°  F.),  should 
be  about  40  mils.  Each  mil  at  this  temperature  represents 
0.0048  gm.  of  pure  amyl  nitrite. 

Sodium  Nitrite  (NaN02  =  69.01).  This,  like  the  other 
nitrites  mentioned,  when  treated  with  potassium  iodid  and 
sulphuric  acid,  is  decomposed,  and  NO  is  given  off.  The 
reaction  is  here  illustrated: 

2NaN02  +  2KI  +  2H2S04 

=  K2SO4  +  Na2S04  +  2H2O  +  2NO  + 12. 

A  molecule  of  NaN02  (69.01)  evolves,  when  properly 
treated,  one  molecule  of  NO  (30.01). 

The  assay  process  is  as  follows:  Weigh  out  0.15  gm.  of 
NaN02,  dissolve  it  in  about  5  mils  of  water,  and  introduce 
the  solution  into  a  nitrometer.     This  is  followed  by  a  solution 


ASSAY  OF  NITRITES  321 

N 
of  I  gm.  of  KI  in  6  mils  of  water  and  15  mils  of  —  H2SO4. 

The  gas  which  is  liberated  should  measure  not  less  than  50 
mils  at  15°  C.  (59°  F.)  or  51.7  mils  at  25°  C.  (77°  F.),  corre- 
sponding to  not  less  than  97.6  per  cent  of  the  pure  salt.  Each 
mil  at  25°  C.  represents  0.002837  &^-  ^^^  ^^  0°  C.  0.0030916 
gm.  of  pure  NaN02. 

Nitric  Acid  in  Nitrates.  This  may  also  be  effected  by  the 
use  of  the  nitrometer. 

When  a  nitrate  is  shaken  up  with  an  excess  of  sulphuric 
acid  and  mercury,  the  nitrate  is  decomposed  and  NO  is  evolved; 
as  seen  in  the  following  equation: 

2KNO3  +  4H2SO4 + 3Hg = 3HgS04  +  K2SO4  +  2NO  +  4H2O. 

2)202.22  2)60.02 

loi.ii  30.01 

Thus  each  molecule  of  the  nitrate  radical  NO3  gives  off  a 
molecule  of  NO. 

Not  more  than  0.2  gm.  of  nitrate  should  be  taken  for 
analysis,  since,  if  this  quantity  is  exceeded,  the  volume  of 
gas  evolved  willl  be  greater  than  the  instrument  can  conve- 
niently hold.  In  this  estimation  the  nitrometer  is  filled  with 
mercury  instead  of  brine;  the  nitrate  is  dissolved  in  5  cc. 
of  water,  introduced  into  the  nitrometer,  and  followed  by 
excess  of  strong  sulphuric  acid.  The  instrument  is  well 
shaken  for  some  time,  and  when  action  has  ceased  and  the 
contents  have  cooled  down  to  the  temperature  of  the  room, 
the  level  is  adjusted  and  the  volume  of  NO  read  off  and  cal- 
culated in  the  usual  way. 


322      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

ESTIMATION    OF   NITROGEN   DIOXID 

NO  =  30.01 ;  I  liter  at    0°  C.  and  760  mm.  =  1.3406  gms. 
at  25°  C.  and  760  mm.  =  1.2281  gms. 

ONE  MILLILITER  OF  NITROGEN  DIOXID  IS  THE  EQUIVALENT  OF: 


Nitrogen  dioxid,  NO  =  30.01 .  .  . 
Amyl  nitrite,  C6HiiN02=  117.10 
Ethyl  nitrite,  C2H6N02=  75.05  . 
Sodium  nitrite,  NaN02  =  69.01 . 


At  0°  C.  and 

760  mm. 

Gram. 


0.0013406 
0.005231 
0.0033528 
0.0030828 


At  25°  C.  and 

760  mm. 

Gram. 


O.OO12281 
O . 0048 
0.003071 
0.002824 


CHAPTER  XX 
HYDROGEN  DIOXID 

As  stated  in  a  previous  chapter,  hydrogen  dioxid  when 
acted  upon  by  an  acidulated  solution  of  potassium  perman- 
ganate, is  decomposed  and  oxygen  is  evolved.  One-half  of 
this  oxygen  comes  from  the  dioxid  and  the  other  half  from 
the  permanganate. 

Therefore  if  i  mil  of  the  dioxid  be  treated  in  this  way 
and  20  mils  of  oxygen  are  evolved,  the  strengjth  of  the  solu- 
tion is  10  volumes. 

The  nitrometer  may  be  used  for  this  estimation. 

This  instrument  is  charged  with  a  concentrated  solution 
of  sodium  sulphate  (which  in  this  case  is  better  than  brine), 
and  I  mil  of  the  dioxid  introduced  from  the  funnel,  followed 
by  excess  of  solution  of  permanganate  acidulated  with  sul- 
phuric acid. 

This  latter  solution  should  be  of  such  strength  that  when 
the  reaction  is  completed,  the  solution  should  still  have  a 
purple  color. 

The  reaction  is  thus  illustrated: 

5H2O2  +  3H2SO4  +  2KMn04  =  K2SO4+ 2MnS04+8H20  +  5O2. 

By  the  use  of  SquihVs  Urea  Apparatus  (Fig.  61)  the 
estimation  may  be  easily  and  rapidly  made. 

Into  the  generating  bottle  is  put  about  30  mils  of  a  strong, 
acidulated  solution  of  potassium  permanganate,  and  a  small 
test-tube  containing  i  mil  of  H2O2  is  carefully  introduced. 
The  two  liquids  must  not  be  allowed  to  come  in  contact. 

323 


324       THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

The  larger  flask  is  filled  with  water,  or  better,  a  solution 
of  sodium  sulphate;  the  connection  is  then  made  by  means 
of  the  rubber  tube,  and  the  generating-bottle  tipped  over  and 
agitated  so  that  the  liquids  will  mix  and  the  reaction  take 
place. 

The  liberated  oxygen  then  passes  into  the  larger  bottle, 
displacing  an  equal  volume  of  water,  which  is  collected  and 
measured.  Half  of  this  volume  represents  the  volume  strength 
of  the  H2O2. 

An  Improvised  Nitrometer  may  be  used.  The  author  has 
found  the  following  instrument  convenient: 

To  the  bottom  of  an  ordinary  50-mil  burette  is  attached 
a  suitable  length  of  rubber  tubing,  to  the  other  end  of  which 
is  attached  another  burette  or  ungraduated  tube,  which  serves 
as  a  control-tube. 

Into  the  top  of  the  burette  is  fitted  a  rubber  stopper, 
through  which  passes  a  short  glass  tube,  which  is  connected 
by  means  of  a  rubber  tube  to  a  generating-bottle  similar  to 
that  used  with  SquihVs  Urea  Apparatus.  Into  the  control- 
tube  is  poured  the  solution  of  sodium  sulphate,  sufficient  to 
fill  the  burette  to  the  zero  mark  and  have  the  surface  of  the 
liquid  in  both  tubes  on  a  level. 

About  30  mils  of  strong  permanganate  solution  acidulated 
with  sulphuric  acid  are  now  placed  in  the  generating-bottle, 
and  then  the  small  test-tube  or  homeopathic  vial,  containing 
exactly  i  mil  of  hydrogen  dioxid,  is  placed  in.  The  gener- 
ating bottle  is  then  stoppered  and  agitated,  the  evolved  gas 
passes  over,  and  forces  down  the  liquid  in  the  burette;  the 
control-tube  is  then  lowered  so  as  to  bring  the  surfaces  of 
the  liquid  in  both  tubes  on  a  level. 

The  reading  is  then  taken. 

Each  mil  of  gas  represents  one-half  volume  of  oxygen 
evolved  from  the  dioxid  if  i  mil  of  the  latter  is  used.     Each  mil 


HYDROGEN  DIOXID  325 

of  oxygen  evolved  from  i  mil  of  the  dioxid  represents  also  0.0017 
gm.  of  absolute  H2O2,  or  0.0008  gm.  of  available  oxygen. 

Thus  if  from  i  mil  of  the  solution  of  hydrogen  dioxid  20 
mils  of  gas  are  evolved,  it  is  a  so-called  ten-volume  solution 
and  contains  0.0017X20=0.034  gm.  of  absolute  H2O2,  or 
0.0008X20  =  0.016  gm.  of  available  oxygen. 

According  to  Naylor  and  Dyer  (Trans.  Brit.  Ph.  Conf., 
1 901,  339)  the  gasometric  permanganate  method  is  unreliable, 
because  under  the  conditions  of  the  test  sulphuric  acid  added 
to  the  brine  solution  in  the  nitrometer  naturally  liberates  a 
little  hydrochloric  acid,  and  this  in  the  presence  of  perman- 
ganate becomes  to  some  extent  decomposed  into  chlorin.  It 
is  the  uncertainty  as  to  the  extent  to  which  the  chlorin  is 
absorbed  by  the  water,  which  renders  the  accuracy  of  the 
method  doubtful.  The  results  of  this  method  are  uniformly 
too  high,  whether  the  gas  be  collected  over  mercury,  over 
saturated  magnesium  sulphate,  or  over  brine,  and  in  the 
latter  case  considerably  higher.  But  when  the  dichromate 
V.S.  is  used  (without  acid),  closely  concordant  results  are 
obtained,  whether  the  gas  be  collected  over  mercury,  or  the 
other  solutions.  The  evolution  of  oxygen  by  the  latter 
method  is  slower  than  when  permanganate  is  used,  but  the 
oxygen  obtained  represents  the  volume  available  in  the  sample. 

In  the  Hypochlorite  Method,  the  nitrometer  is  filled  with 
a  saturated  solution  of  sodium  chlorid.  Two  mils  of  the  hydro- 
gen dioxid  are  admitted  into  the  measuring  tube,  the  funnel 
tube  filled  with  a  little  water,  and  this  also  let  in,  then  20  mils  of 
the  chlorinated  lime  solution  introduced.  From  this  point  the 
procedure  is  the  same  as  in  the  gasometric  permanganate  method. 

Ca(C10)2  +  2H2O2  =  O4  +  CaCl2  +  2H2O. 

The  presence  of  preservatives,  except  inorganic  ones,  gives 
low  results. 


326 


THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 


The  Hypobromite  Method.  W.  M.  Dehn  (J.  A.  C.  S., 
XXIX  (9),  13 15)  describes  an  accurate  and  rapid  determination 
of  hydrogen  dioxid  by  means  of  sodium  hypobromite,  using 
a  ureometer.     The  reaction  involved  is 

H2O2  +  NaBrO  =  NaBr  +  H2O  +  Oo. 


H^ 


.-A 


_--C 


The  apparatus  is  shown  in  Fig.  58.  The  following  descrip- 
tion of  the  method  is  by  Dehn  from  the 
Journal  of  the  American  Chemical  Society. 
"  The  stop-cock  E  is  opened  and  the  stop- 
cocks D  and  F  are  closed;  then  the  solution  of 
sodium  hypobromite  *  is  poured  in  at  the  top 
of  C  until  it  fills  the  tubes  A  and  C  to  some 
point  above  the  stop-cock  E. 

"  The  stop-cock  E  is  then  closed  and  the 
stop-cock  F  is  opened,  so  that  the  hypobro- 
mite in  C  may  run  down  to  the  constricted 
portion;  the  solution  in  A  is  then  sustained 
by  atmospheric  pressure.  The  stop-cock  D 
(arranged  so  as  to  deliver  only  in  the  two 
directions  of  a  right-angle  triangle)  is  turned 
from  the  position  shown  in  the  figure  and  is 
so  controlled  that  B  may  first  be  washed  with 
a  little  of  the  hydrogen  peroxid  and  then  be 
filled  with  the  same  to  a  readable  height  on 
the  scale.  Upon  turning  D  so  as  to  admit  a 
regulated  volume  of  the  hydrogen  peroxid  solution  an  immediate 
evaluation  of  oxygen  results.  After  admitting  most  of  the  hypo- 
bromite held  above  E,  and  letting  stand  for  a  minute  or  two 
so  as  to  drain  properly,  the  columns  of  hypobromite  in  A 

*  This  solution  is  prepared  as  directed  under  Estimation  of  Urea,  except 
that  it  is  finally  diluted  with  an  equal  volume  of  water. 


Fig.  58. 


HYDROGEN  DIOXID 


327 


and  C  are  brought  to  the  same  level,  the  volume  of  oxygen 
is  read  off  and  its  weight  and  that  of  the  corresponding 
hydrogen  dioxid  are  calculated  by  the  usual  formulas." 

The  author  of  this  method  also  gives  the  following  table, 
by  means  of  which  the  mils  of  oxygen,  under  various  conditions 
of  temperature  and  pressure,  may  be  calculated  into  milli- 
grams of  hydrogen  dioxid,  and  claims  that  by  the  use  of  this 
instrument,  this  hypobromite  method  and  the  table  for  calcu- 
lating the  assay  of  hydrogen  dioxid,  is  not  only  rapid  and 
accurate,  but  the  necessity  of  preparing  and  correcting  standard 
solutions  is  avoided  and  the  presence  of  the  usual  preserva- 
tives used  in  the  dioxid  solution  may  be  ignored. 


WEIGHT 

IN  MILLIGRAMS 

OF  H2O2 

CORRESPONDING 

TO  ONE 

CUBIC 

CENTIMETER   OF 

MOIST 

OXYGEN 

t/mvn.. 

728 

732 

736 

740 

744 

748 

4° 

1.2664 

1-2734 

1.2802 

1.2872 

1.2942 

1.3011 

8 

I . 2463 

2531 

I . 2600 

2669 

1.2736 

2805 

12 

I. 2251 

2317 

1.2387 

2454 

1.2522 

2589 

i6 

I . 2044 

2111 

I. 2178 

2245 

1.2311 

2378 

20 

1.1817 

1884 

I . 1948 

2015 

I . 2080 

2145 

24 

'    I. 1583 

1649 

1.1719 

1777 

1.1843 

1907 

28 

I -1345 

1411 

I. 1476 

1538 

I . 1603 

1665 

32 

I . 1085 

1 149 

1.1213 

1275 

I -1338 

1401 

36 

I .0843 

0905 

1.0967 

10^0 

I . 1093 

1155 

40 

I .0605 

0666 

1.0725 

.0786 

1.0849 

0909 

i/mva. 

752 

756 

760 

764 

768 

4° 

I. 3081 

I-3151 

1.3222 

1.3290 

1-3359 

8 

1.2876 

1-2944 

1.3014 

I. 308 I 

3150 

12 

1.2657 

1.2726 

1.2823 

I . 2860 

2928 

16 

I . 2444 

I. 2512 

1.2578 

I . 2946 

2713 

20 

I. 2213 

1.2279 

1-2345 

1.2410 

2475 

24 

I. 1972 

1 . 2036 

I. 2100 

1.2169 

2230 

28 

1-^731 

I. 1796 

1.1857 

1.1922 

1986 

32 

I . 1465 

1. 1528 

1.1589 

1.1562 

1715 

36 

1.1214 

1-1279 

1.1341 

1.1402 

1465 

40 

1.0971 

I -1033 

I . 1094 

I-I155 

1216 

CHAPTER  XXI 

ESTIMATION  OF  SOLUBLE  CARBONATES  BY  THE  USE  OF 
THE  NITROMETER 

The  nitrometer  may  be  used  for  estimating  ammonium 
carbonate  in  aromatic  spirit  of  ammonia. 

The  nitrometer  in  this  case  must  be  charged  with  mercury, 
as  the  liberated  CO2  is  soluble  in  aqueous  liquids. 

A  given  volume  of  the  spirit  is  introduced  into  the  nitrometer 
followed  by  an  excess  of  dilute  HCl,  and  the  evolved  gas  then 
read  off;  and  from  its  quantity  the  proportion  of  ammonium 
carbonate  may  be  calculated  by  applying  the  equation 

(NH4)2C03  +  2HCI  =  2NH4CI  +  H2O  +  CO2. 

♦96  *44 

The  volume  of  gas  liberated  must  first  be  reduced  to  its 
corresponding  volume  at  0°  C. 

Each  mil  of  CO2  at  0°  C.  weighs  0.001966  gm.  Now  if 
44  gms.  of  CO2  represent  96  gms.  of  normal  ammonium  car- 
bonate, how  much  ammonium  carbonate  does  0.001966  gm. 
of  CO2  represent? 

44:96: :  0.001966  :x.         :x;  =  0.004289  gm. 

Thus  each  mil  of  CO2  at  normal  pressure  and  0°  C. 
represents  0.004289  gm.  of  (NH4)2C03,  approximately. 

*  The  atomic  weights  are  approximate. 

328 


CHAPTER  XXII 

ESTIMATION  OF  UREA  IN  URINE 

The  determination  of  urea  is  based  upon  the  fact  that 
when  urea  is  decomposed  by  an  alkaline  hypochlorite  or 
hypobromite,  carbon  dioxid  and  nitrogen  are  given  off,  as 
the  equation  shows: 

CO  (NH2)  2  +  3NaBr  O  =  3NaBr  +  CO2  +  N2  +  2H2O. 

The  liberated  N  may  be  measured,  and  from  its  quantity 
the  amount  of  urea  calculated.  The  other  products  of  the 
decomposition  go  into  solution. 

The  hypobromite  solution  is  prepared  as  follows:  100 
gms.  NaOH  are  dissolved  in  250  mils  of  water,  and  when  this 
solution  has  become  cold  25  mils  of  bromin  are  added,  and 
the  solution  kept  cold.  This  solution  contains  sodium  hypo- 
bromite, bromate  and  hydroxid.  It  readily  undergoes  decom- 
position, and  should  therefore  always  be  freshly  prepared 
when  wanted  for  use.  To  15  mils  of  the  NaOH  solution  add 
I  mil  of  bromin. 

The  solution  of  sodium  hypochlorite  is  generally  preferred 
to  the  hypobromite,  because  it  is  more  stable,  just  as  effica- 
cious, and  the  disagreeable  handling  of  bromin  is  obviated. 

Various  forms  of  apparatus  have  been  devised  for  the 
quantitative  estimation  of  urea. 

The  Doremus  Ureometer  (Fig.  59)  is  the  simplest  of  these. 
The  long  arm  of  the  ureometer  is  filled  with  the  hypobromite 
solution,  and  then  i  mil  of  the  urine  is  introduced  by  the  aid 
of  the  pipette.     The  pipette  is  introduced  through  the  bulb 

329 


V 


330       THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 


as  far  as  it  will  go  in  the  bend,  and  the  nipple  is  then  gently 
but  steadily  compressed,  being  careful  that  no  air  is  admitted. 

The  volume  of  the  liberated  gas  is  read  off  after  the  froth 
has  subsided. 

The  ureometer  indicates,  according  to  its  graduation,  either 
milligrams  of  urea  in  i  mil  or  grains  of  urea  per  fluid  ounce 
of  urine. 

It  also  indicates  by  the  signs   +,  N,  and  —   whether  the 


^^^\ 


Fig.  59. 


Fig.  60. 


urea  is  present  in  an  increased,  normal  or  decreased  quantity. 
Either  Knop's  or  Squibb's  solution,  or  Liquor  Sodae  Chloratae 
U.  S.  P.  may  be  used  in  this  instrument.  Knop's  solution  is 
that  described  above.  Squibb's  solution  contains  potassium 
bromid  as  well  as  bromin.  It  is  prepared  by  taking  an  equal 
weight  of  bromin  and  of  potassium  bromid,  and  adding  eight 
times  as  many  cc.  of  water  as  there  were  grams  of  bromin 
taken.  For  use  mix  equal  volumes  of  this  solution  with  the 
sodium  hydroxid  solution  above  described. 


ESTIMATION  OF  UREA  IN  URINE 


331 


The  Hinds-Doremus  Ureometer.  This  apparatus,  which 
is  shown  in  Fig.  60,  is  capable  of  giving  more  exact  results 
than  the  original  apparatus,  because  the  i  mil  of  urine  can 
be  delivered  more  accurately.  It  consists  of  a  bulb  with  an 
upright  tube  a,  graduated  like  the  original,  so  that  each  of 
the  smallest  divisions  represents  o.ooi  gm.  of  urea  in  the  urine 
used.  The  lower  portion  of  this  tube  is,  in  connection  with 
a  smaller  tube  c,  graduated  with  a  capacity  of  2  mils;  between 
these  tubes  a  glass  stop-cock  is  situated.  Closing  the  stop- 
cock h,  Knop's  or  Squibb's  fluid  (diluted  one  half)  or  liquor 
sodae   chloratae  U.  S.  P.  is  introduced    into  tube  a  so   as  to 


Fig.  61. 


completely  fill  it.  The  apparatus  is  then  placed  in  an  upright 
position  and  the  smaller  tube  c  is  filled  to  the  zero  mark  with 
urine.  The  stop-cock  is  then  turned  slowly  so  as  to  admit 
gradually  i  mil  of  the  urine  to  tube  a.  After  fifteen  minutes 
the  reading  is  taken.  If  the  reading  be  0.015  and  the  amount 
of  urine  taken  was  i  mil,  then  multipl3;ing  by  100  gives  1.5 
per  cent  of  urea. 

Squibb's  Urea  Apparatus  (Fig.  61)  is  a  very  simple  appa- 
ratus, and  can  be  easily  improvised  in  a  drug  store.  It  con- 
sists of  two  wide-mouthed  bottles,  the  larger  of  which,  C, 
capable  of  holding  about  250  mils,  is  fitted  with  a  rubber  stopper, 
through  which  is  passed  a  curved  delivery  tube  and  a  short 
straight  tube,  the  latter  connected  by  a  piece  of  rubber  tubing 


I 


332      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

to  the  short  glass  tube  in  the  rubber  stopper  of  the  smaller 
or  generating-bottle  B.  In  the  genera  ting-bottle  is  a  small 
test-tube  A . 

Into  the  test-tube  A  is  placed  5  mils  of  urine,  and  into  the 
smaller  bottle  B  is  put  20  mils  of  the  hypobromite  solution, 
or  strong  liquor  sodae  chloratae.  The  test-tube  is  then  placed 
in  the  generating-bottle  B,  being  careful  that  the  urine  and 
the  reagent  do  not  come  in  contact.  The  larger  bottle  C  is 
now  filled  with  water  and  the  two  bottles  connected  by  the 
rubber  tube,  the  larger  bottle  being  placed  on  its  side  upon 
a  block,  and  when  all  connections  are  tight,  the  generating- 
bottle  is  shaken  so  that  the  urine  will  mix  with  the  reagent. 

Decomposition  takes  place,  and  the  generated  gas  passes 
into  the  bottle  C,  displacing  water,  which  is  caught  in  a  grad- 
uated cylinder  or  other  measuring  vessel.  The  volume  of 
water  displaced  is  equivalent  to  the  volume  of  gas  evolved. 

Each  mil  of  nitrogen  gas  evolved  at  0°  C.  and  normal 
pressure  represents  0.C0268  gm.  of  urea.  Then  by  multi- 
pljiing  the  number  of  mils  evolved  by  this  number  the  quantity 
of  urea  in  the  5  mils  of  urine  taken  is  ascertained. 

The  volume  of  gas  obtained  when  the  operation  is  con- 
ducted at  ordinary  temperatures  should  always  be  reduced 
to  its  corresponding  volume  at  0°  C.  and  760  mm. 

The  factor  0.00268  is  found  in  the  following  manner: 

1000  mils  of  H  *at  0°  C.  weigh  0.0896  gm. ; 
1000  mils  of  N  at  0°  C.  weigh  1.255  g^^s. 

By  the  equation  it  is  seen  that  60.02  gms.  of  urea  evolve 
when  decomposed  28.02  gms.  of  N. 

CO  (NH2)  2 + 3NaBrO  =  3NaBr  -^  CO2  +  N2  +  2H2O. 

60.02  28.02 


ESTIMATION  OF  UREA  IN  URINE  333 

Now  we  will  find  the  volume  occupied  by  28.02  gms.  of 
N  at  0°  C. 

1.255  gms.  of  N=iooo  mils. 

gms.       mils        gms.  mils 

1.255  :  1000::  28.02  :  X.        x  =  22^26. 

Thus  60.02  gms.  of  urea  evolve  22326  mils  of  N.     One 
mil  of  N  thus  represents  0.00268  gm.  of  urea. 


APPENDIX 

DESCRIPTION  OF  INDICATORS 

The  following  list  includes  the  more  reliable  indicators  in 
common  use,  arranged  alphabetically. 

A I  i  7^  r i  n  •     alkalies  =  Red 
Alizarin.     ^^.^^      =  Yellow 

This  dye  was  first  found  in  the  roots  of  madder  (Rubia 
tinctorium)  but  is  now  also  obtained  synthetically.  A  half 
per  cent  solution  in  alcohol  is  employed  as  an  indicator. 

AzoHtmin:    '^^Z^ 

This  is  the  color  principle  to  which  litmus  owes  its  value 
as  an  indicator.  Its  extraction  is  explained  under  litmus. 
It  is  a  high-priced  article  and  is  in  consequence  seldom  used, 
the  purified  litmus  tincture  bemg  preferred. 

Brazil  Wood  Solution:    ^^^f^^=P^rpiish-red 

acids      =  Yellow 

Boil  50  gms.  of  finely-cut  Brazil-wood  (the  heart-wood 
of  Peltophorum  dubium  (Sprengel)  Briton,  nat.  ord.  Leguminosce) 
with  250  mils  of  water  during  half  an  hour,  replacing  from 
time  to  time.  Allow  the  mixture  to  cool;  strain;  wash  the 
contents  of  the  strainer  with  water  until  100  mils  of  strained 
liquid  are  obtained;  add  25  mils  of  alcohol  and  filter. 

This  indicator  is  especially  serviceable  for  the  titration  of 

334 


I 


DESCRIPTION  OF  INDICATORS  335 

alkaloids,  but  it  is  useless  in  the  presence  of  sulphurous  acid, 
sulphites,  or  sulphids,  as  these  substances  decolorize  the  solu- 
tion. 

Cochineal:    S'-Z Sti./,-.. 

Cochineal  is  the  dried  female  insect,  Pseudococcus  cacti 
Linne. 

The  test  solution  is  made  by  macerating  3  gms.  of  unbroken 
cochineal  for  four  days  in  250  mils  of  a  mixture  of  i  part  of 
alcohol  and  3  parts  of  water,  by  volume.  It  should  be  neu- 
tralized with  ammonia  water  before  using. 

It  is  a  very  valuable  indicator,  especially  for  carbonates 
of  the  alkalies  and  alkaline  earths,  because  it  is  not  affected 
by  CO2.  It  is  also  useful  for  titrating  alkaloids,  alkalies, 
alkali  earths,  ammonia,  and  inorganic  acids,  but  is  useless 
for  most  organic  acids. 

Congo  Red:    ^""1^^ 

Congo  red  is  a  sodium  tetrazo-diphenyl-naphtionate.  It 
occurs  in  commerce  in  the  form  of  lumps  of  a  reddish-brown 
color.  It  is  readily  soluble  in  both  water  and  alcohol.  Its 
solution  is  exceedingly  sensitive  to  free  acids  even  in  the  presence 
of  acid  salts,  and  is  likewise  very  sensitive  to  free  carbonic 
and  acetic  acids.  It  may  be  employed  for  estimating  free 
mineral  acids,  in  the  presence  of  most  organic  acids. 

The  test  solution  contains  i  per  cent  of  the  dye  and  lo 
per  cent  of  alcohol. 

nallf^iiT     alkalies  =  Br*^;i/re<f 
Uduem.    ^^.^g     ^  Pale  brown 

Anthracene  violet  or  pyrogallol-phthalein  was  proposed  by 
M.  Dechan  for  use  as  an  indicator. 

It  is  prepared  by  heating  a  mixture  of  one  part  o^  phthalic 


336       THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

anhydrid  and  two  parts  of  pyrogallol,  and  finally  recrys- 
tallizing  in  a  similar  way  to  phenolphthalein. 

It  is  described  as  a  dark  reddish  crystalline  solid,  possessing 
a  greenish  luster.  It  is  nearly  insoluble  in  water,  but  readily 
soluble  in  alcohol.  In  commerce  it  is  frequently  found  as  a 
paste,  mixed  with  water. 

It  forms  a  red  coloration  with  alkalies,  which  is  changed 
to  yellowish-brown  on  addition  to  an  acid  in  excess. 

It  is  said  to  be  more  delicate  toward  alkalies  than  phenol- 
phthalein, and  may  be  used  in  its  stead  for  titrating  many 
of  the  alkaloids.  It  may  be  used  in  the  presence  of  ammonia 
or  its  salts.  It  indicates  sharply  with  the  organic  acids.  A 
solution  in  rectified  spirit  i-iooo  is  generally  employed. 

M»^»«»^^-^iri:»  .     adka,\[es=  Violet 
HaematOXylin:     ^^^^      =  Yellow  to  orange 

A  peculiar  principle  obtained  from  logwood,  having  the 
composition  C16H14O6,  and  crystallizing  with  one  or  three 
molecules  of  water.  It  is  an  efflorescent  yellowish-rose  colored 
substance,  but  when  pure  is  said  to  be  colorless,  reddening 
on  exposure  to  light. 

It  is  soluble  in  hot  water  or  alcohol.  Its  alcoholic  solution 
is  largely  used  as  an  indicator  in  the  titration  of  alkaloids, 
for  which  it  is  considered  the  indicator  par  excellence. 

The  solution  is  prepared  by  dissolving  0.3  gm.  of  the  well- 
crystallized  material  in  100  mils  of  alcohol.  In  titrating  use 
about  three  drops  of  this  solution. 

Its  color  reaction  with  carbonates  and  bicarbonates  is 
interesting.  When  added  to  a  solution  of  alkali-bicarbonate 
the  reaction  requires  many  seconds,  and  results  in  a  gradually 
deepening  carm.ine-red  which  is  permanent,  while  in  the  case 
of  soluble  carbonates  the  reaction  is  instantaneous,  a  purple- 
red  which  changes  rapidly  through  cherry,  eosin-red  to  orange. 


DESCRIPTION  OF  INDICATORS  337 

The  reaction  with  ammonrum  carbonate  is  similar  to  that 
with  bicarbonate. 

lodeosin:    S^IfX^" 

Tetra-iodo-fltu)rescein  Ery  thro  sin  B.  This  indicator  is  use- 
ful for  minute  quantities  of  alkali,  as  for  instance,  such  as 
may  be  dissolved  out  from  glass  on  contact  with  water.  It 
is  used  in  connection  with  highly  dilute  standard  solutions 
only.  The  iodeosin  solution  is  made  by  dissolving  0.002  gm. 
of  the  indicator  in  1000  mils  of  pure  ether.  Titration  with  this 
indicator  is  carried  out  by  introducing  50  to  100  mils  of  the 
liquid  to  be  titrated  into  a  stoppered  bottle  and  adding  10  to 
20  mils  of  the  ethereal  indicator  solution  and  setting  aside 
after  shaking.  The  ethereal  layer  as  well  as  the  fluid  will 
be  colorless  if  the  latter  is  neutral,  but  if  traces  of  alkali  are 
present  the  rose-red  tint*  passes  into  the  aqueous  liquid  leaving 
the  ether  colorless.  If  the  fluid  is  acid,  the  ethereal  layer  is 
yellow.  If  preferred,  4  or  5  drops  of  a  1-10,000  alcoholic 
solution  of  the  indicator  may  be  added  to  the  liquid,  and 
ether  then  added.  lodeosin  is  particularly  useful  in  titration 
of  alkaloids,  especially  those  of  weak  basicity,  as  emetine, 
atropine,  morphine,  etc. 

Lacmoid:    ^^Z^ 

Lacmoid  is  somewhat  allied  to  litmus,  but  differs  from 
it  in  many  respects.  It  is  a  product  of  resorcin,  and  may 
be  prepared  by  heating  gradually  to  110°  C.  a  mixture  of 
100  parts  of  resorcin,  5  parts  of  sodium  nitrite,  and  5  parts 
of  water.  After  the  violent  reaction  moderates  it  is  heated 
to  120°  C.  until  ammonia  ceases  to  be  evolved.  The  residue 
is  then  dissolved  in  warm  water  and  the  lacmoid  precipitated 
therefrom  by  HCl;  the  free  acid  is  then  removed  by  washing 
and  the  residue  dried. 


338       THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

This  constitutes  the  commercial  lacmoid,  which  is  not  in 
a  sufficiently  pure  state  to  be  used  as  an  indicator;  its  puri- 
fication is  effected  according  to  Forster  by  treating  the  powder 
with  boiling  water  and  acidulating  the  resulting  blue  solution 
with  hydrochloric  acid.  After  a  few  hours  the  precipitate  is 
collected  and  washed  with  a  little  cold  water  and  carefully 
dried,  or  it  is  dissolved  in  alcohol  and  the  solution  evaporated. 
Even  after  careful  purification,  lacmoid  solution  may  still 
exhibit  a  violet  tinge,  which  is  a  disturbing  factor  in  accurate 
work.  To  remedy  this  defect,  Forster  suggests  the  addition 
of  5  gms.  of  beta-naphthol  green  to  3  gms.  of  purified  lacmoid 
dissolved  in  700  mils  of  water  and  300  mils  of  alcohol.  A 
sample  of  lacmoid  which  is  only  sparingly  soluble  in  water 
should  be  rejected.  The  purer  the  article  the  more  readily 
does  it  dissolve  in  water.  A  good  T.S.  may  be  made  by 
dissolving  i  part  in  12  parts  of  20  per  cent  alcohol,  filtering 
and  evaporating  in  vacuo  over  sulphuric  acid.  Of  the  residue 
obtained  in  this  way  0.2  gm.  is  dissolved  in  100  mils  of  alcohol. 

Lacmoid  paper  is  prepared  by  dipping  slips  of  calendered 
unsized  paper  into  the  blue  or  red  solution  and  drying  them. 

Lacmoid  is  slightly  affected  by  carbonic-acid  gas.  It  may 
be  used  cold  for  the  alkaline  and  earthy  hydroxids,  arsenites, 
and  borates  and  the  mineral  acids.  The  carbonates  and 
bicarbonates  of  the  alkalies  and  alkali  earths  are  titrated 
hot  with  this  indicator. 

Many  of  the  metallic  salts,  such  as  the  sulphates  and 
chlorids  of  iron,  copper  and  zinc,  which  are  more  or  less 
acid  to  litmus,  are  neutral  to  lacmoid;  therefore  free  acids 
in  such  solutions  may  be  estimated  by  its  aid. 

Lacmoid  paper  reacts  alkaline  with  the  chromates  of  potas- 
sium or  sodium,  but  neutral  with  the  dichromates,  so  that 
a  mixture  of  the  two  or  of  chromic  acid  and  dichromate  may 
be  titrated  by  its  aid. 


DESCRIPTION  OF  INDICATORS  339 

Litmus  {Lacmus):    S'"l^'- 

A  pigment  obtained  by  the  fermentation  of  certain  lichens, 
principally  from  Roccella  tinctoria  and  R.  fuciformiSj  but  also 
from  other  species  of  lichen. 

It  occurs  in  commerce  in  small-,  friable,  light  cakes  or 
cubes,  of  a  violet  color. 

The  coloring  piinciples  of  litmus  are  azolitmin,  erythro- 
litmin,  and  erythrolein.  The  first,  which  is  the  most  important, 
is  soluble  in  water,  but  insoluble  in  alcohol.  The  other  two 
are  readily  soluble  in  alcohol,  but  only  sparingly  soluble  in 
water. 

The  process  for  making  litmus  test  solution  consists  in 
exhausting  coarsely  powdered  litmus  with  boiling  alcohol. 

The  residue  is  then  digested  with  about  an  equal  weight 
of  cold  water  so  as  to  dissolve  the  excess  of  alkali  present. 

The  blue  solution  thus  obtained,  after  being  acidulated, 
may  be  used  to  make  red  litmus-paper.  Finally,  the  residue 
is  extracted  with  about  five  times  its  weight  of  boiling  water 
and  the  solution  filtered. 

The  filtrate  is  preserved  as  test  solution  in  wide-mouthed 
bottles,  stoppered  with  loose  plugs  of  cotton  to  exclude  dust, 
but  to  admit  air. 

When  kept  in  closed  vessels  litmus  solution  gradually 
loses  color,  but  this  returns  upon  exposure  to  air  and  conse- 
quent absorption  of  oxygen. 

The  fermentation  to  which  the  loss  of  color  is  due  may 
be  prevented  by  saturating  the  solution  with  NaCl  or  by  the 
addition  of  thymol  or  phenol. 

The  British  Pharmacopoeia  recommends  the  boiling  of  litmus 
in  powder  with  three  succcessive  portions  of  rectified  spirit, 
and  then  to  digest  the  residue  in  distilled  water  and  filter, 
the  object  of  these  steps  in  the  process  being  to  get  rid  of 


340        THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

the  greater  portion  of  erythrolitmin  and  erythrolein,   which 

are  soluble  in  alcohol.     Then  by  treating  the  residue  with 

water  a  larger  proportion  of  azolitmin  is  dissolved,  and  the 

solution  is   contaminated  with  very   little  of  the  other,  two 

principles. 

Litmus  test  solution  should  be  of  such  strength  that  3 

drops  added  to  50  mils   of  water  will  impart  to  the  latter  a 

N 
distinct  color.     If  one  drop  of  —  acid  or  alkali  solution  be 

added  to  this  the  color  should  change  to  red  or  blue. 

Litmus  may  be  used  in  a  very  large  number  of  titrations. 
It  is  of  value  in  the  titration  of  most  mineral  acids  and  of  a 
few  organic  acids,  e.g.,  benzoic  and  oxalic.  It  is  also  useful 
in  the  titration  of  alkaline  hydroxids  when  the  latter  are  free 
from  carbonates. 

But  for  carbonates,  bicarbonates,  etc.,  a  reliable  end-reaction 
can  only  be  obtained  by  boiling  the  solution  during  the  titra- 
tion, in  order  to  dispel  the  liberated  CO 2. 

Free  CO2  has  an  acid  reaction  with  litmus,  and  interferes 
very  much  with  the  finding  of  the  end-reaction. 

Litmus  may  be  used  for  ammonia  and  for  borax.  It  is 
of  no  use  for  phosphoric  or  arsenic  acid,  nor  for  sulphurous 
acid,  phosphates,  or  arsenates,  because  the  change  of  tint 
is  too  gradual. 

It  is  unsatisfactory  in  titrating  many  organic  acids,  e.g., 
tartaric  and  citric,  but  may  be  used  for  oxalic  or  benzoic, 
as  before  stated. 

Sometimes  it  is  required  to  perform  a  titration  with  litmus 
at  night.  Gas-  or  lamp-light  is  not  adapted  for  showing  the 
reaction  satisfactorily,  but  by  using  a  monochromatic  light, 
such  as  the  sodium  flame,  a  very  sharp  line  of  demarcation 
may  be  found. 

The  operation  should  be  conducted  in  a  dark  room,  using 


DESCRIPTION  OF  INDICATORS  341 

a  piece  of  platinum  foil  sprinkled  with  salt  or  a  piece  of  pumice- 
stone  saturated  with  a  solution  of  salt,  heated  in  a  Bunsen 
flame. 

The  red  color  then  appears  perfectly  colorless,  while  the 
blue  appears  like  a  mixture  of  ink  and  water. 

I  iif  Ar»l  •     alkalies  =  Fe//ow 
uuieoi.     ^^jjg     =  Colorless 

Chemically  it  is  an  oxy-chlor-diphenyl-quinoxalin.  It  was 
suggested  as  an  indicator  by  Autenrieth. 

The  solution  for  the  purpose  of  an  indicator  is  prepared 
by  dissolving  i  part  in  loo  parts  of  alcohol.  Of  this,  four 
drops  are  sufficient  for  50  mils  of  fluid  to  be  titrated. 

In  sensitiveness,  luteol  exceeds  both  litmus  and  phenol- 
phthalein.  It  is  more  sensitive  toward  ammonia  than  Nessler's 
solution.  Ten  mils  of  a  solution  containing  one  drop  of  am- 
monia water  per  liter,  is  colored  yellow  immediately  upon  add- 
ing luteol,  whereas  with  Nessler's  solution  it  takes  quite  some 
time  before  a  reaction  is  obtained. 

Methyl  Orange:    Sf '=L'f  "^ 

Poirrier's  Orange  III,  Tropseolin  D,  Helianthin,  Mandarin- 
orange,  Para-sulpho-benzeneazo-dimethylanilin. 

This  is  prepared  by  the  action  of  diazo-sulphanilic  acid 
upon  dimethylanilin;  the  acid  so  formed  is  converted  into 
a  sodium  or  ammonium  salt,  purified  by  reprecipitation  with 
HCl,  and  again  converted  into  a  sodium  or  ammonium  salt. 
If  prepared  carefully  and  from  the  purest  materials,  it  is  a 
bright  orange-red  powder,  perfectly  soluble  in  water  and 
slightly  in  alcohol;  but  it  is  often  found  in  commerce  as  a 
dull  orange-brown  powder,  often  not  completely  soluble  in 
water.    Many  conflicting  statements  have  been  made  by  opera- 


342       THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

tofs  as  to  the  value  of  methyl  orange  as  an  indicator,  which 
have  tended  to  bring  this  indicator  into  disrepute. 

Sutton  has  examined  many  specimens,  but  has  not  found 
any  in  which  the  impurities  sensibly  affected  its  delicate  action. 
He  claims  that  the  common  error  is  the  use  of  too  much 
indicator,  and  that  some  eyes  are  more  sensitive  to  a  change 
of  tint  than  others. 

Methyl  orange  is  no  doubt  a  very  good  indicator,  but 
practice  with  it  must  be  had  in  order  to  obtain  good  results. 
The  author  has  found  one  sample  which  had  a  beautiful 
orange  color,  but  which  was  absolutely  useless  as  an  indicator. 

A.  H.  Allen  describes  the  characters  and  tests  of  a  good 
article  as  follows: 

1.  Aqueous  solution,  not  precipitated  by  alkalies.  (Orange 
I  becomes  red-brown;   orange  II  brownish-red.) 

2.  Hot  concentrated  aqueous  solution  yields  with  HCl 
microscopic  acicular  crystals  of  the  free  sulphonic  acid,  soon 
changing  to  small  lustrous  plates  or  prisms  having  a  violet 
reflection.  (Orange  I  gives  yellow-brown  color  or  flocculent 
precipitate;  orange  II  brown-yellow  precipitate.) 

3.  Dissolves  in  concentrated  H2SO4  with  a  reddish  or 
yellowish-brown  color,  which  on  dilution  becomes  fine  red. 

4.  BaCl2  yields  a  precipitate. 

5.  CaCl2  yields  no  precipitate.  (Orange  I  gives  a  red 
precipitate.) 

6.  Pb(C2H302)2  yields  an  orange-yellow  precipitate. 

7.  MgS04  in  dilute  solutions  precipitates  the  coloring 
matter  in  microscopic  crystals. 

Methyl-orange  T.S.  is  made  by  dissolving  i  gm.  of  methyl 
orange  in  1000  mils  of  water.  Add  to  it  carefully  diluted  sul- 
phuric acid  in  drops  until  the  liquid  turns  red  and  just  ceases 
to  be  transparent.     Then  filter. 

The  great  value  of  this  indicator  consists  in  the  fact  that 


DESCRIPTION  O^  INDICATORS  343 

it  is  not  affected  by  carbonic-acid  gas,  sulphureted  hydrogen, 
or  silicic,  oleic,  stearic  and  many  other  acids. 

It  answers  well  for  ammonia,  but  it  is  useless  for  most 
of  the  organic  acids.  Phosphoric  and  arsenic  acids  are  rendered 
neutral  to  methyl  orange  when  only  one  third  of  the  acid  has 
•combined  with  the  base,  the  end-reaction  being  well  defined. 
(Phenolphthalein  indicates  neutrality  when  two-thirds  of  acid 
are  combined.) 

This  indicator  should  not  be  employed  when  titrating  with 
standard  solutions  which  are  weaker  than  decinormal,*  nor 
should  it  be  used  in  any  hot  titrations,  nor  in  excessive  quan- 
tities. Two  drops  are  sufficient  for  50  mils  of  the  fluid  to  be 
titrated,  or  just  enough  to  give  it  a  faint  tint. 


Methyl  Red:    ^^f^^fow 

•^  acid    =Rose  red 

{Paradimethylaminoazohenzene  -  orthocarhoxylic  acid)  — ■ 
(CH3)2  -N  -C6H4  -N  =  NC6H4  •  COOH.  Violet  crystals,  very 
slightly  soluble  in  cold  water,  yielding  a  red  solution,  readily 
soluble  in  alcohol  and  glacial  acetic  acid.  When  dissolved  in 
sulphuric  acid  it  shows  the  same  color  change  as  methyl  orange. 
The  applications  of  methyl  red  as  indicator  are  the  same  as 
those  of  methyl  orange,  except  in  the  case  of  titration  of  car- 
bonates. 

Methyl  Red  T.S.  Dissolve  0.2  gm.  of  methyl  red  in  100 
mils  of  alcohol.  Its  end-points  are  sharper  than  those  of 
methyl  orange,  and  it  is  far  more  sensitive  for  dilute  solutions 
of  weak  bases,  surpassing  hematoxylin  or  iodeosin  in  the 
titration  of  alkaloids. 


344      THE  ESSENTIALS  OF  'VOLUMETRIC  ANALYSIS 

„    ,.  r  carbonates  =  jRei 
Phenacetolin :     ^^^^^  \  hydroxids  =  Yellow 
acids  =  Yellow 

This  indicator  is  prepared  by  heating  together  for  several 
hours  equal  molecular  weights  of  phenol,  glacial  acetic  acid, 
and  sulphuric  acid  in  a  vessel  provided  with  a  reflux  con- 
denser. The  product  is  then  thoroughly  washed  with  water 
to  remove  excess  of  acid  and  dried  for  use.  It  is  only  very 
slightly  soluble  in  water,  but  dissolves  readily  in  alcohol, 
forming  a  greenish-brown  solution. 

The  solution  yields  with  alkali  hydroxids  a  scarcely  per- 
ceptible pale  yellow,  but  with  normal  carbonates  of  the  alkalies, 
sulphids,  and  with  ammonia  it  gives  a  decided  pink  color; 
with  bicarbonates  a  more  intense  pink,  while  with  acids  a 
golden  yellow. 

This  indicator  is  useful  for  estimating  the  amount  of  alkali 
or  alkali  earth  hydroxids  in  the  presence  of  carbonate,  unless 
the  hydroxid  is  present  in'  too  small  a  quantity.  Ammonia 
must  not  be  present.  The  titration  is  carried  out  by  adding 
the  acid  until  a  faint  red  color  appears;  this  indicates  that 
the  alkali  hydroxid  or  the  lime  has  been  neutralized.  The 
further  addition  of  the  acid  intensifies  the  red  until  the  car- 
bonate present  in  the  mixture  is  neutralized,  when  a  golden- 
yellow  color  appears.  The  proportion  of  alkali  hydroxid  must 
be  far  in  excess  of  the  carbonate  in  order  to  t)btain  reliable 
results;  furthermore,  considerable  practice  is  required  in  the 
use  of  this  indicator  in  order  to  accustom  the  eye  to  the  color 
changes. 

A  convenient  strength  of  solution  is  i :  loo  in  alcohol. 


DESCRIPTION  OF  INDICATORS  345 

Phenolphthalein  (C^oHmO^):    '^fZ^lLie. 

Preparation.  Five  parte  of  phthalic  anhydrid  (C8H4O3), 
10  parts  of  phenol  (CeHeOH),  and  4  parts  of  H2SO4  are 
heated  together  at  120°  to  130°  C.  for  several  hours.  The 
product  is  then  boiled  with  water,  and  the  residue,  which 
consists  of  impure  phenolphthalein,  is  dissolved  in  dilute 
soda  solution  and  filtered.  By  neutralizing  this  solution  the 
phenolphthalein  is  precipitated  and  may  be  purified  by  crys- 
tallization from  alcohol;  or  the  alcoholic  solution  may  be 
boiled  with  animal  charcoal,  filtered,  and  the  phenolphthalein 
reprecipitated  by  boiling  water. 

Uses.  Phenolphthalein  is  a  very  valuable  indicator;  is 
extremely  sensitive,  and  exhibits  a  well-marked  and  prompt 
change  from  colorless  to  pink,  and  vice  versa.  A  few  drops 
of  the  solution  of  the  indicator  show  no  color  in  neutral  or 
acid  liquids,  but  the  faintest  excess  of  alkali  produces  a  sudden 
change  to  red. 

It  may  be  employed  in  the  titration  of  mineral  and  organic 
acids  and  most  alkalies,  but  it  is  not  suited  for  the  titration 
of  ammonia  or  its  salts.  It  is  very  sensitive  to  CO2,  and 
therefore  in  estimating  carbonates  the  liquid  must  be  boiled, 
as  with  litmus.  It  is  inapplicable  for  borax,  except  in  the 
presence  of  glycerin,  because  the  color  gradually  fades  away 
as  the  acid  is  added.  One  great  advantage  which  phenol- 
phthalein possesses  is  that  its  indications  may  be  clearly 
read  in  many  colored  liquids;  another  is  that  it  may  be  used 
in  alcoholic  liquids  or  in  mixtures  of  alcohol  and  ether,  and 
therefore  many  organic  acids  which  are  insoluble  in  water 
may  be  accurately  titrated  by  its  help. 

Phenolphthalein  T.S.  is  a  one  per  cent  solution  in  alcohol. 


346      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

■    ,,    y  f  ca,ThonaXes  =  Blue 

Poirrier  Blue  (C4B):    ""'^^'^ \  hydroxids  =Red 

acids  ==Blue 

This  indicator,  which  is  closely  allied  to  Gentian  Blue  in 
properties,  is  obtained  by  the  a,ction  of  sulphuric  acid  on 
triphenylrosanilin.  It  is  a  blue  powder  with  a  coppery  luster. 
It  dissolves  in  water  and  in  alcohol,  yielding  blue  solutions. 
KOH  and  NaOH  change  the  color  to  red,  but  ammonia 
decolorizes  it.  It  is  employed  as  an  indicator  in  aqueous 
solution  1:500.  This  indicator  is  exceedingly  sensitive  to 
g,cids.  Borax  and  boric  acid  give  a  blue  color;  in  the  titra- 
tion of  boric  acid  the  red  color  does  not  appear  until  the  acid 
is  completely  neutralized.  This  indicator  is  recommended 
for  the  titration  of  hydrocyanic  acid,  toward  which  it  is  especially 
sensitive,  the  alkaline  cyanids  are  alkaline  in  reaction  to  most 
indicators,  but  C4B  does  not  show  an  alkaline  reaction  until 
the  HCN  is  completely  neutralized,  and  a  minute  excess  of 
the  alkali  hydroxid  has  been  added.  C4B  is  of  the  character 
of  a  weak  acid  and  its  salts  are  very  unstable;  they  are  decom- 
posed by  water  alone  when  in  very  great  dilution,  therefore 
the  indicator  must  be  used  in  sufficient  quantity.  The  addition 
of  a  few  drops  of  alcohol  facilitates  the  color  change,  which 
is  indeed  a  very  sharp  one. 

Resazurin:    f^^Z^:; 

This  is  a  new  indicator  for  alkalimetry,  proposed  by  Crismer. 
It  is' prepared  as  follows:  Dissolve  4  gms.  of  resorcin  in  300 
mils  of  anhydrous  ether  and  add  40  to  45  drops  of  nitric  acid 
(sp.gr.  1.25)  saturated  with  nitrous  anhydrid.  Allow  the 
mixture  to  stand  in  a  cold  place  for  two  days,  whereupon 
a  deposit  of  blackish  crystals,  having  a  reddish-brown  reflec- 
tion, will  be  formed  in  the  bottom  of  the  vessel.     The  super- 


DESCRIPTION  OF  INDICATORS  347 

natant  clear  red  liquid  is  decanted  and  the  crystals  washed 
with  ether  until  the  washings  show  a  blue  color  with  ammonia 
water. 

Resazurin  (C12H7NO4)  is  slightly  soluble  in  water,  more 
so  in  alcohol  and  freely  soluble  in  acetic  ether.  It  produces 
a  blue  solution  with  water,  alkalies,  and  alkali  carbonates, 
which  are  turned  red  upon  the  addition  of  a  slight  excess  of 
acid.  To  use  this  indicator  in  alkalimetry,  Crismer  recom- 
mends the  following  solution:    Resazurin  0.2   gm.  dissolved 

N  .  .  . 

in  40  mils  of  —  ammonia  solution,  and  made  up  to  1000  mils 
10  ^  r 

with  distilled  water. 

This  is  deep  blue  in  color  and  keeps  well.  Two  or  three 
drops  are  sufficient  to  color  200  mils  of  liquid. 

This  indicator  is  not  suited  for  the  titration  of  nitric  acid 
or  monobasic  organic  acids,  and  it  is  not  very  sensitve  to 
carbonic  acid.  It  is,  however,  extremely  sensitive  to  alkalies. 
If  the  solution  is  acidulated  to  a  rose-red  color  and  heated 
in  a  white  glass  flask,  the  solution  will  turn  blue  through 
the  alkaline  reaction  of  the  dissolved  glass  before  the  boiling- 
point  is  reached. 

This  indicator  is  especially  useful  for  borax. 

Rosolic  Acid  (C20HX4O3):    '^t'Z^l^ 

This  compound  is  also  called  Aurin  and  Coralline,  and 
is  prepared  as  follows: 

A  mixture  of  phenol  and  sulphuric  acid  is  placed  upon 
a  water-bath,  and  oxalic  gradually  added,  waiting  each  time 
till  the  evolution  of  gas  ceases,  and  using  less  oxalic  acid  than 
is  requu-ed  to  attack  all  the  phenol. 

In  this  process  the  oxalic  acid  is  decomposed  into  CO, 


348      THE  ESSENTIALS  OF  VOLUMETRIC  ANALYSIS 

CO2,  and  H2O.    The  CO  immediately  reacts  with  the  phenol 
and  forms  rosolic  acid,  as  the  following  equation  shows: 

3C6H5OH  +  2CO  =  C20H14O3  +  2H2O. 

Commercial  rosolic  acid  is  a  mixture  of  several  derivatives 
among  them  the  above,  methylaurin  C20H16O3  and  others. 
Commercial  poeonin  (also  known  as  Aurin  R.)  [chiefly  C19H14O3] 
may  be  used  in  place  of  rosolic  acid. 

Rosolic  acid  is  soluble  in  diluted  alcohol.  Its  color  is 
pale  yellow,  not  changed  by  acid,  but  turns  violet-red  with 
alkalies. 

It  is  an  excellent  indicator  for  the  mineral  acids  and  strong 
bases,  weak  ammoniacal  solutions,  oxalic  acid  and  other  or- 
ganic acids,  except  acetic. 

The  test  solution  is  made  by  dissolving  i  gm.  of  the  com- 
mercial rosolic  acid  in  10  mils  of  diluted  alcohol  and  then  adding 
engugh  water  to  make  100  mils. 

Tropaeolin  (OO):    Sf  ^Z  i::^- ,,.,^ 

This  is  used  in  the  form  of  a  solution  containing  0.5  gm. 
to  1000  mils  of  alcohol. 

Turmeric  Tincture.  Digest  any  convenient  quantity  of 
ground  curcuma-root  (from  Curcuma  longa  Linne,  nat.  ord. 
ScUaminece)  repeatedly  with  small  quantities  of  water,  and 
throw  this  liquid  away.  Then  digest  the  dried  residue  for 
several  days  with  six  times  its  weight  of  alcohol  and  filter. 

Turmeric  Paper.  Impregnate  white,  unsized  paper  with 
the  tincture  and  dry  it. 

The  color  principle  of  turmeric  is  curcumin.  It  is  seldom 
used  in  volumetric  analysis,  except  in  the  form  of  turmeric 
paper.     For  high-colored  solutions  curcumin  gives  no  reaction 


DESCRIPTION  OF  INDICATORS  349 

with  acids,  but  becomes  brown  with  alkalies.  There  is  another 
color  principle  in  turmeric  besides  curcumin,  which  is,  however, 
useless  in  that  it  is  indifferent  to  alkalies;  it  is  soluble  in  water, 
and  extracted  by  digestion  with  water,  after  which  the  cur- 
cumin  is  dissolved  out  with  alcohol. 

Turmeric  paper  is  especially  useful,  because  of  its  peculiar 
reaction  with  boric  acid,  with  which  it  develops  a  brown  color 
after  drying,  and  which  color,  when  touched  with  caustic 
soda  solution  is  changed  to  dark  green. 


INDEX 


Acetate,  lead 169 

—  potassium 85 

—  sodium 86 

Acetic  acid 108 

Acetone 306 

Acid,  acetic 108 

—  arsenous 197 

decinormal  V.S 241 

solution  of 198 

V.S.,  use  of  in  reduction , 240 

—  defined 58 

—  benzoic 109 

—  boric 108 

—  chromic 176,  229,  237 

—  citric 109 

—  hydriodic '. 1 23 

—  hydriodic  by  sulphocyanate  method 125 

—  hydriodic,  syrup  of 1 26 

—  hydrobromic 122 

by  sulphocyanate  method 122 

by  Volhard's  Method ^H^- ^ ^^ 

using  chromate  as  indicator .^P^r 125 

—  hydrochloric 104 

normal 63 

action  of,  on  permanganate 148 

standardization  of 67 

standardization  by  sodium  carbonate 65 

—  hydrocyanic 133 

using  chromate  indicator 132 

potassium  iodid  indicator 133 

—  hypophosphorous 107,  164 

—  lactic 110 

• —  nitric 107,  321 

351 


352  INDEX 

PAGE 

Acid,  nitrous 163 

—  number  of  resins 290 

—  oxalic 159 

and  oxalates 159 

decinormal 63 

—  phosphoric 105,  128 

—  rosolic 18,  339 

—  salicylic 109 

—  sulphuric 104 

normal 67 

—  sulphurous 201 

—  tartaric 109 

—  trichloracetic 109 

—  value 277 

-of  fats  and  oils 279 

Acidimetry ' , 95 

—  and  alkalimetry 59 

Acids,  estimation  of,  by  neutralization 95 

—  haloid 1 24 

—  inorganic 102 

—  organic ic8 

—  quantity  to  be  taken  for  assay 104 

•     —  weighing  of,  for  assay 101 

volatile 10^ 

Alcohol,  in  tinctures  and  beverages • 308 

Alcoholometric  table 309 

Alizarin 334 

.--    Alkali  bicarbonates  and  carbonates  mixed 78 

—  carbonates 71 

—  hydroxid  and^Bjtotoate  mixed 77 

—  hydroxid,  stj^BPl^rotion,  preservation  of 97 

—  hydroxids,  esmnation 67 

—  iodids 230 

—  standard  solutions,  preparation  of 97 

\       Alkali  earth  hydroxids 90 

and  carbonates  mixed 93 

salts QOj  94 

—  earths,  organic  salts  of 81 

—  organic  salts  of 80 

Alkalimetry 61 

—  and  acidimetry 59 

Alkaloids,  estimation  of 251 


r 

INDEX  353 

PAGE 

Alkaloids,  extraction  of 259 

—  separation  of 260 

Ammonia,  aromatic  spirits 328 

—  water 70 

stronger 71 

Ammonium,,  benzoate 89 

—  carbonate 74 

—  salicylate : .  89 

Amyl  nitrite .320,  322 

Anions , 19 

Anthracene  violet 335 

Antimonic  compounds 200,  238 

Antimonous  oxid 199 

Antimony  compounds 199 

—  and  potassium  tartrate 200 

Apparatus,  cleaning  of 40 

—  used  in  volumetric  analysis 29 

—  use  of 40 

Arsenates 238 

Arsenic  oxid 238 

Arsenic  trioxid 197 

Arsenite  of  potassium  solution •. 198 

Arsenous  acid 197 

solution , 198 

decinormal  V.S 240 

V.S.,  use  of  in  reduction .  240 

—  anhydrid 197 

—  compounds 196 

direct  percentage  assay  of 198 

—  iodid /•.•.-i(^ 198 

—  oxid ^^\. 196 

standardization  of  iodin  with 191 

Atomic  weights xii 

multiples  of xiii 

Azolitmin 334 

Barium  chlorid 112 

—  dioxid 158,  162 

—  hydroxid  V.S loi 

—  peroxid 162 

—  soluble  salts  of 90 

Benzoate,  ammonium 89 


354  INDEX 

PAGE 

Benzoate,  sodium 86 

Benzoic  acid 109 

Berzelius'  system  of  oxids 56 

Bicarbonate  of  sodium ; 74 

—  of  potassium 73 

Bisulphite,  sodium • 205 

Bitartrate,  potassium 85 

Bleaching  powder 219,  244 

Borax 79 

Boric  acid 108 

Boyle's  Law 315 

Brazil  wood 334 

test  solution 18,  334 

Bromates 234 

Bromate,  potassium 235 

Bromids 119,  122 

Bromin  free 217 

—  V.S 271 

—  water 222 

Burette,  automatic 31 

—  connected  with  reservoir 32 

—  clamps • 34 

—  glass  stop-cock 30 

—  holder 34 

—  Mohr's 29 

Burettes,  special  forms  of 45 

Burette  supports 34 

Butter,  examination  of 281 

Cacodylate,  sodu^^ 79 

Calcium  carbonat^^ 91,  166 

—  chlorid 93 

—  lactate 87 

—  salts 166 

—  soluble  salts  of 166 

Calculating  results 50 

Calibration  of  instruments 46 

Calx  chlorinata 219 

Cane  sugar,  inverted 292 

Carbonate,  ammonium , ....  74 

—  and  hydroxid  of  alkali  mixed 77 

—  calcium 163 


INDEX  355 

PAGE 

Carbonate,  of  lithium 74 

—  potassium 72 

—  of  sodium  (anhydrous) 74 

(crystallized) 73 

—  sodium,  normal  V.S 92 

Carbonates  and  bicarbonates  of  alkalies,  mixed 78 

Carbonates  and  hydroxids  of  alkali  earths,  mixe" 93 

—  of  alkalies 71 

—  soluble,  assay  of  by  the  use  of  the  nitrometer 328 

Cathions 19 

C4B 345 

Centinormal  solutions 10 

Charles'  Law 315 

Chloral  hydrate 94 

Chlorate  potassium 175,  235 

Chlorates 175,  234 

Chlorid,  calcium » 93 

—  ferric 237 

—  of  Hme 219 

—  sodium , 116,  120,  123 

V.S 116 

Chlorids 1 20,  123 

Chlorin,  in  bleaching  powder 219,  244 

—  in  chlorin  water ,  ..217,  237,  243 

—  free 217,  237 

—  water • 217,  237,  243 

Chlorinated  lime 219,  238,  244 

—  soda  solution 221 

Chlorometry 240 

Chromates f . .  .   176,  229,  237 

Chromic  acid 176,  229,  237 

—  anhydrid ^  .  176,  229 

—  oxids 176,  229 

Chromium  trioxid 229 

Chromophoric  theory 21 

Citrate  lithium 86 

—  potassium 85 

Citric  acid 109 

Cochineal 18,  335 

Coefficients  for  caclulating  analyses 53 

Cold  way,  titration 72 

Congo  red 335 


356  INDEX 

PAGE 

Copper  assay 238 

Cream  of  tartar 83 

Cyanid  potassium 134 

Cyanogen 131 

Cylinder,  graduated 39 

Decinormal  solutions 10 

Diastasic  value  of  malt  extract 297 

Bichromate,  potassium 229,  237 

analysis  by  means  of 179 

preparation  of  V.S 180 

Digestion  methods 232 

Dioxid,  hydrogen 158,  222,  323 

—  manganese ; 1 70,  2 27,  245 

Direct  percentage  estimations 50 

table  of  quantities  for 1 1 1 

Dissociation  theory 19 

Distillation  methods 223 

Double  normal  solutions 11 

Eau  de  Javelle 221 

Elements,  list  of xii 

Empirical  permanganate  solutions,  use  of 150 

—  solutions II 

End-reaction.  . , 17,  113 

Eosin 18 

Erdmann's  float 44 

Erythrosin  B 337 

Factors S3 

Fats,  waxes  and  oils 277 

Fehling's  solution ....'.  291 

end-point 294 

Ferric  alum  solution •. 122 

—  chlorid 237 

—  salts 209,  233,  237 

Ferrous  ammonium  sulphate 146 

—  carbonate  saccharated 184 

—  iodid,  syrup  of 127 

—  salts,  estimation  by  dichromate 182 

—  sulphate 154,  186 

Ferrum  reductum i55 


INDEX  357 

PAGE 

Flask,  liter 38 

—  measuring 38 

Flasks,  titration 39 

Fluorescein • 19 

Formaldehyde 299 

Free  fatty  acids 277 

Galenical  preparations 265 

Gallein .335 

Gasometric  analyses 313 

Gay-Lussac's  method  for  haloid  salts 1 20 

General  methods  of  assaying  drugs 263 

General  principles.  .* 4 

Glucose •• 292 

Glycerophosphate,  sodium •. .  .  79 

Gordin's  modified  alkalimetric  assay 257 

Gravimetric  method,  the i 

Grethan's  pipette 103 

Grouvelle's  bleaching  fluid 221 

Haematoxylin 336 

Halogens,  free 242 

Haloid  acids 1 24 

—  salts 118 

estimation  of  with  chromate  as  an  indicator 118 

Mohr's  method 118 

Hanus'  number 288 

Helianthin 341 

Hot  way  titrations 71 

Hiibl's  number 285 

Hydriodic  acid ^^ 125 

syrup ^P 126 

Hydrobromic  acid •  .  122,  1 25 

Hydrochloric  acid 104 

—  —  action  of,  on  permanganate 148 

normal 63 

standardization  of 65 

Hydrocyanic  acid 133 

Hydrogen  dioxid 158,  222,  323 

Hydrogen  sulphid 206 

Hydroxid  and  carbonate  of  alkali  mixed 77 

of  alkali  earths,  mixed 93 


358  INDEX 

PAGE 

Hydroxid,  potassium » 69 

—  potassium  normal  V.S 97 

—  sodium 70 

normal  V.S 100 

Hydroxids,  alkali,  estimation 67 

—  of  alkali  earths 90 

—  sodium  and  potassium  mixed 78 

Hypobromite  solution  for  urea  estimation 329 

Hypochlorite 221 

Hypophosphite,  calcium 130 

Hypophosphites 130,  164 

Hypophosphorous  acid 107,  126,  164 

Hyposulphite,  sodium .* 206 

V.S.  preparation  of 210 

Immiscible  solvents 260 

Indicator 1 7,  61 

Indicators,  classification  of 25 

—  description  of  individual 334 

—  guide  for  the  selection  of 27 

Indicator,  requirements  of  a  good '. 27 

—  sensitiveness  to  alkaloids. 270 

—  theories  of 21 

Indirect  oxidation,  analysjj^by 187 

Inorganic  acids ^^ 102 

Instruments,  calibration  of.  !l^ 46 

Introduction i 

Inverted  cane  sugar 292 

lodates 233,  234 

lodeosin 337 

lodid,  arsenous.  .^^ 198 

—  ferrous  syrup  dUPf.. 127 

—  potassiuA'. 123 

lodids 117,  119 

—  alkali 230 

lodin,  free 215 

—  purification  of 189 

—  tincture 213 

—  titrations,  use  of  sodium  bicarbonate  in 194 

Iodine  absorption  number 285,  288 

—  V.S.  preparation  of 189 

Iodized  starch  test  paper 242 


INDEX  359 

PAGE 

lodometry 209 

lodometric  estimations,  indirect 216 

Ionization  theory 19 

Ions 19 

Iron,  estimation  of  by  stannous  chlorid 247 

—  reduced 152 

Javelle's  water 221 

Katz's  method 267 

Kebler's  Keller  method '. 265 

Kingzett's  method 222 

Knop's  hypobromite  solution 330 

Kappeschaar's  solution *. 271 

Kottstorfer  number 278 

Labarraque's  solution 221 

Lacmoid 337 

Lacmus 339 

Lactate,  calcium 87 

Lactic  acid no 

Lactose 292 

Law  of  Boyle 315 

—  of  Charles 315 

Lead  acetate 169 

—  peroxid ' 233 

—  salts \ 168 

—  subacetate 168 

Lime,  chlorid  of 219 

—  chlorinated 219 

Liter  flasks 38 

Lithium  carbonate ^. 74 

—  citrate ^P^ ^^ 

—  organic  salts  of 81 

Litn.us 339 

—  tincture  of 18,  339 

Lloyd's  method 265 

Lugol's  solution 2i(5 

Lunge's  pipette 103 

Luteol 341 

Magnesium  carbonate  and  hydroxid 93 

—  sulphate,  use  of  in  permanganate  titrations 149 


360  INDEX 

PAGE 

Malt  extract,  diastasic  value  of ,   297 

maltose  in 295 

Maltose 292,  295 

Mapdarin-orange 341 

Manganates 209,  223 

Manganese  dioxid 1 70,  227,  245 

Manganous  sulphate,  use  of  in  permanganate  titrations 149 

Measuring  flask 38 

Meniscus 43 

Mercuric  salts 136,  239,  249 

—  sulphate,  use  of,  in  permanganate  titrations 149 

Mercurous  salts 239 

Methyl  orange , 18,  341 

—  red 343 

Mil 14 

Milliliter 14 

Mohr's  burette 29 

—  salt 149 

Multiples  of  atomic  weights xiii 


Neutralization  analysis 58 

—  of  acids " 95 

Nitrate,  silver  V.S 114 

Nitrates,  nitric  acid  in 321 

—  Pelouze  method 171 

Nitric  acid io7;  173,  321 

Nitrite,  amyl 320,  322 

—  ethyl 318,  322 

—  sodium 320 

Nitrites 163,  317 

Nitrogen  dioxid. ^^ 322 

Nitrometer,  the.^^ ....  313 

Nitrous  acid 163 

—  ether 318 

Normal  oxalic  acid 62 

—  solutions 7 

Orange,  methyl 341 

Oils,  fats  and  waxes 277 

—  iodin  absorption  number  of .". , 285,  288 

—  table  showing  iodin  absorption  number  of 290 


INDEX  361 

PAGE 

Organic  acids io8 

—  salts  of  alkalies 80 

of  the  alkalies,  table  of  factors  for 87 

of  alkali  earths 83 

of  lithium 83 

Oxalic  acid 159 

and  oxalates 159 

decinormal 63 

normal  solution 62 

standardization  of  permanganate  by 144 

Oxid,  antimonous 198 

— »arsenous 197 

Oxidation  and  reduction 139 

—  indirect  analysis  by 187 

Oxy-chlor-diphenyl-quinoxalin 341 

Paraformaldehyde 306 

Paranitrophenol 23 

Perborate,  sodium 162 

Percarbonates 163 

Percentage  rules  for  direct  estimations 50 

Permanganate,  action  of  hydrochloric  acid  on 148 

—  potassium  V.S 141 

—  solutions,  empirical 150 

—  titration  with  in  presence  of  hydrochloric  acid 149 

—  typical  analysis  with 154 

—  volumetric  analysis  by  means  of 146 

Peroxids '. 209,  223 

Phenacetolin 343 

—  in  estimating  mixed  alkali  hydroxid  and  carbonate 77 

Phenol 273-275 

Phenolphthalein 18,  345 

Phosphate,  sodium 1 29 

Phenylsulphonates 276 

Phosphoric  acid 105,  1 28 

Pinch  cocks 33 

Pipettes • 34 

Poirrier  blue 345 

Poirrier's  orange  iii 341 

Potassium  acetate 85 

—  arsenite  solution 198 

—  bicarbonate 73 


362  INDEX 

PAGE 

Potassium  bi-iodate,  advantages  of  for  standardizing  V.S loo 

preparation  of loo 

—  bitartrate 85 

purification  of 99 

—  bromate 235 

—  bromid 122 

—  carbonate 72 

—  chlorate ; 175,  235 

—  chromate  T.S 19 

—  citrate 85 

—  cyanid 134 

—  dichromate 4^29 

—  ferricyanid  T.S 19 

—  hydroxid • . .  .  69 

normal  V.S 97 

-^  —  alcoholic j^, .  270 

—  iodid 119 

—  permanganate  V.S 141 

—  sodium  tartrate 83 

—  sulphite 205 

—  -sulphocyanate  V.S 117 

—  tartrate 8i 

Precipitation  analysis 113 

Preparation  of  normal  oxalic  acid  solution 62 

—  of  standard  acid  solutions 61 

ProUius'  fluid 260 

Puckner's  method 264 

Pyrogallol-phthaleiA 335 

Ramsay's  bleaching  fluid 221 

Reading  of  instruments 42 

Reduced,  estimation  of  substances  readily 208 

—  iron 156 

Reducing  agents .^ 208 

—  sugars 292 

Reduction  and  oxidation  analysis 139 

—  methods,  involving  the  use  of  arsenous  acid  V.S 240 

stannous  chlorid 246 

Reichert  number 279 

Reichert-Meis§l*!ftmber 280 

Resazurin 346 

Residual  titration. '. 15 


INDEX  363 


Resins,  acid  number  of 290 

Resorcinol 276 

Re-titration 15 

Rochelle  salt 83 

Rosolic  acid 18,  347 

Rules  for  direct  percentage  estimations 50 

—  for  finding  percentage 54 

Salicylate,  ammonium 89 

—  sodium 86 

—  strontium 87 

Salicylic  acid 109 

Saponification  number 278 

Seidlitz  powder 87 

Seminormal  solutions 10 

Separators 261 

Shaking-out  process  for  alkaloids 260 

Silver  alloys 135 

—  metallic 135 

Silver  nitrate,  assay  of  by  means  of  sodium  chlorid  V.S 134 

sulphocyanate 135 

V.S 114 

—  salts 134 

Soda,  chlorinated 221 

Sodium  acetate )% 86 

tartrate 83 

—  benzoate 86 

—  bicarbonate .* 74 

use  of  in  titrations  with  iodin 194 

—  bisulphite 206 

—  borate 79 

—  cacodylate • yg 

—  carbonate  (anhydrous) 74 

(crystallized) , 73 

normal  V.S 92 

pure,  how  to  make 92 

—  chlorid 1 20 

preparation  of  pure 116 

V.S ^.. 116 

— ■  glycerophosphate 79 

—  hydroxid 70 

V.S 100 


364  INDEX 

PAGE 

Sodium  hyposulphite ; 206 

V.S.,  preparation  of 210 

—  nitrite 320 

—  perborate 162 

—  phosphate 1 29 

—  salicylate 86 

—  sulphite 205 

—  tetrazo-diphenyl-naphthonate 335 

—  thiosulphate 206 

V.S.,  preparation  of 210 

Spirit  of  ammonia 71 

aromatic 328 

—  of  nitrous  ether 318 

Squibb's  hypobromite  solution 330 

Standard  solutions 7 

—  temperature 13 

Stannous  chlorid,  estimation  of  iron  by  means  of 247 

solution,  preparation  of 247 

use  of  in  reduction  methods 246 

Starch 292 

—  as  an  indicator 192 

—  after  inversion 295 

—  inversion  by  diastase 296 

—  iodized,  test  paper 242 

—  solution K , 192 

Stating  results 55 

Strontium  salicylate 87 

Subacetate,  lead . .  : 169 

Sugars 291 

Sugar  in  urine 293 

Sulphid,  hydrogen 206 

Sulphids,  insoluble 207 

Sulphuric  acid 104 

normal 67 

Sulphite,  potassium 205 

—  sodium ; 205 

Sulphites .  .^ 201 

Sulphocariblates 276 

Sulphocyanate  method 1 20 

—  potassium  V.S 117 

Sulphogranate  V.S.,  assay  of  silver  nitrate  by 134 

Sulphim)us  acid 201,  203 


INDEX  365 

PAGE 

Syrup  of  hydriodic  acid 126 

—  of  ferrous  iodid 127 

Table,  alcoholometric 309 

—  for  correction  of  volume  for  the  temperature 42,  317 

—  for  correction  of  pressure 3^17 

—  of  elements xii 

—  of  factors  of  organic  salts  of  the  alkalies 89 

—  of  normal  factors  for  acids,  alkalies  and  alkali  earths 55 

—  of  normal  factors  for  oxids,  etc 57 

—  of  multiples xiii 

—  of  quantities  for  direct  percentage  estimations no 

—  of  substances  estimated  by  standard  iodin  solution 208 

precipitation 137 

permanganate  and  dichromate 187 

—  showing  color  changes  of  indicators 256 

factors  for  alkaloids 256 

iodin  absorption  number  of  oils 288 

saponification  number 273 

Tartar,  cream  of 83 

—  emetic ■ , 200 

Tartaric  acid 109 

Tartrate,  antimony  and  potassium 200 

—  potassium 81 

—  sodium  and  potassium 8;^ 

Temperature,  standards  of 13 

Test  mixer 39 

Tetra-iodo-fluorescein 337 

Theories  of  indicators * 21 

Theory,  chromophoric 21 

—  ionization  of  indicators 21 

—  of  Ostwald W 21 

Thiosulphate  sodium 206 

V.S.,  estimations  involving  use  of 209 

preparation  of 210 

—  standardization  of  iodin  with 190 

—  V.S.,  standardization  of  by  dichromate..  ^ 211 

of  by  potassium  bi-iodate 213 

of  by  iodin 210 

of  by  permanganate _  ,   215 

Titrate,  to , 14 

Titrated  solution 7 


366  INDEX 

PAGE 

Titration,  influence  of  concentration  of  V.S 41 

of  rate  of  speed 41 

of  temperature 41 

—  residual 15 

Titer 7 

Trichloracetic  acid 109 

Tropaeolin  D • 341 

—  (0  0) 348 

Turmeric 18 

—  paper 348 

—  tincture : . , 348 

Urea  apparatus,  Squibb's 331 

—  eLtimation  of  by  Doremus'  ureometer 329 

Ureometer,  Doremus' 329 

—  Hinds-Doremus' 331 

Urine,  sugar  in 293 

Valence 12 

Vegetable  drugs,  assaying  of 259 

Violet,  anthracene ^ 335 

Volatile  acids,  weighing  of 103 

—  fatty  acid,  value 279 

—  solvents,  influence  of  in  alkaloidal  assays 270 

Volhard's  method  for  haloids 1 20 

—  solution 117 

Volume  strength  of  hydrogen  dioxid 162 

Volumetric  method,  the 2 

—  or  standard  solutions 7 

Weighing  bottle 102 

Wilson's  ble^ing  fluid 221 

Zinc  salts. . . , , , 93 


Wiley  Special  Subject  Catalogues 

For  convenience  a  list  of  the  Wiley  Special  Subject 
Catalogues,  envelope  size,  has  been  printed.  These 
are  arranged  in  groups — each  catalogue  having  a  key 
symbol.  (See  special  Subject  List  Below).  To 
obtain  any  of  these  catalogues,  send  a  postal  using 
the  key  symbols  of  the  Catalogues  desired. 


1— Agriculture.    Animal  Husbandry.    Dairying.     Industrial 
Canning  and  Preserving. 

2 — ^Architecture.      Building.      Masonry. 

3 — Business  Administration  and  Management.    Law. 

Industrial  Processes :   Canning  and  Preserving;    Oil  and  Gas 
Production;  Paint;  Printing;  Sugar  Manufacture;  Textile. 

CHEMISTRY 

4a  General;  Analytical,  Qualitative  and  Quantitative;  Inorganic; 
Organic. 

4b  Electro-  and  Physical;  Food  and  Water;  Industrial;  Medical 
and  Pharmaceutical;  Sugar.' 

CIVIL  ENGINEERING 

5a  Unclassified  and  Structural  Engineering. 

5b  Materials  and  Mechanics  of  Construction,  including;  Cement 
and  Concrete;  Excavation  and  Earthwork;  foundations; 
Masonry. 

5c  Railroads;  Surveying. 

5d  Dams;  Hydraulic  Engineering;  Pumping  and  Hydraulics;  Irri- 
gation Engineering;  River  aod  Harbor  Engineering;  Water 
■Supply.  ^ 

(Over) 


CIVIL  KNGINEEKING— Continued 
5e  Highways;     Municipal    Engineering;     Sanitary     Engineering; 
Water    Supply.      Forestry.      Horticulture,    Botany    and 
Landscape  Gardening. 


6 — Design.       Decoration.       Drawing:     General;     Descriptive 
Geometry;  Kinematics;  Mechanical. 

ELECTRICAL  ENGINEERING— PHYSICS 
7 — General  and  Unclassified;  Batteries;  Central  Station  Practice; 
Distribution  and   Transmission;  Dynamo-Electro  Machinery; 
Electro-Chemistry  and   Metallurgy;   Measuring     Instruments 
and  Miscellaneous  Apparatus. 


8 — Astronomy.      Meteorology.      Explosives.      Marine    and 
Naval  Engineering.     Military.     Miscellaneous  Books. 

MATHEMATICS 
9 — General;    Algebra;  Analytic  and  Plane   Geometry;   Calculus; 
Trigonometry;  Vector  Analysis. 

MECHANICAL  ENGINEERING 
10a  General  and  Unclassified;  Foundry  Practice;  Shop  Practice. 
10b  Gas  Power  and   Internal   Combustion  Engines;  Heating  and 

Ventilation ;  Refrigeration . 
10c  Machine  Design  and  Mechanism;  Power  Transmission;  Steam 

Power  and  Power  Plants;  Thermodynamics  and  Heat  Power. 
11 — ^Mechanics.  

12 — Medicine.  Pharmacy.  Medical  and  Pharmaceutical  Chem- 
istry. Sanitary  Science  and  Engineering.     Bacteriology  and 

Biology. 

MINING  ENGINEERING 

13 — General;  Assaying;  Excavation,  Earthwork,  Tunneling,  Etc.; 
Explosives;  Geology;  Metallurgy;  Mineralogy;  Prospecting; 
Ventilation.  # 


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STAMPED  BELOW 

ration  of  loan  period^^ r^^^======= 


3r»-8,'38(3(i29s) 


421! 


